Guide:Ideas

So, you've completed the endgame and mastered your world. There are still many options for playing with your current world, or making the next game far more challenging:

Projects
You probably already built some of the building projects, try upgrading them. However, if you didn't build one in the current world you are playing in, try them now! Feel free to browse the Terraria forums for ideas and inspiration.
 * Hellevators for rapid travel upwards and downwards.
 * Skybridges and railways for similar travel across the map.
 * Mushroom & Plant Farms.
 * Battle Arenas.
 * Traps and other structures for farming monsters.
 * Shooting Range: Connect one or more Bird Statues (or even other statues including enemies) to a Timer or a Switch. Then use a gun (Musket, Minishark, Shotgun, Blowpipe) to shoot at the spawned creatures.  Make it more challenging by adding a few obstacles in the arena (possibly Active Stone Blocks connected to a Timer?) and close off the arena to prevent the prey from escaping.
 * Multi-boss fight: If you have defeated every boss in Terraria, try summoning and fighting multiple bosses at once.
 * Make complex devices with Mechanisms, Wiring, and Hoiks.
 * Kill the Dungeon Guardian
 * Fishing Challenges! Try to collect one of every fish in the game.

Art
Terraria's sandbox offers vast opportunities for creativity. Reshape your map and create varied structures: You can range from elegant simplicity to intricate complexity, realistic to fantastic, and from the beautiful to the terrifying or hideous. Again, the forums can provide ideas
 * NPC Houses: Maybe you created tiny Houses for your Non Player Characters to live in and now want to improve them. What better way to do this than to make custom houses for all your NPCs? Some simple decorations can quickly turn any normal house into one that fits the theme of the NPC. For example add some Plants and a Flower Wall to the house of your Dryad. Maybe some Bottles and a bed for your nurse? A Crystal Ball for your Wizard and an Ammo Box for your Arms Dealer. Some Christmas decoration for Santa Claus and so on. All NPCs have some items that suit their theme, allowing you to build some nice houses for all your NPCs.
 * Castle: One of the most popular constructions in Terraria is a castle. Of course you can build a simple symmetric castle, but this often tends to look rather boring. A good way to build a more interesting castle is to start by building some tower tops of varying sized and varying heights and after that connect them. For colors the Gray Bricks and Red Bricks combine well, while some more expensive bricks may look good for the throne room. Aside from that use your own imagination and build the castle the way you like it.
 * Mine: Not only is it handy to have direct access to a large mine but it looks great too. Using some Wooden Beams, Planked Walls and torches you can make a simple yet good looking mine for yourself.
 * Tree House: Another popular construction. You can either build your own tree house or modify a Living Tree to create one. Make sure to use some more natural building blocks and walls to make it look better.
 * Outposts: Another popular construction is to build outposts in the different biomes, usually in the style of the biome it is build in. This also allows you to easily buy items that NPCs only sell in specific biomes (especially if you use Teleporters or a King/Queen Statue to teleport the NPCs there).
 * Themed Houses: There are many furniture themes in Terraria. For example take a look at the different Doors that are available. You can make a house (or other structure) for each of those themes.
 * Pyramid: Although some Pyramids can be found in the world they are hardly worthy as a tomb for an emperor. Make a giant above ground pyramid, half maze and half trapped to make a worthy burial chamber.
 * Domes: You want easy access to all different biomes, but you don't want to leave your house? Make the biomes come to you. Build large glass domes with a biome inside of every dome.
 * Treasure Chamber: You may not have noticed yet, but you can place Coins, Bars and Gems. Although generally a waste to do so, this can be used to make a great looking treasure chamber.
 * Atlantis: Yet another popular construction. You can build cities basically everywhere. Below the ground, in the skies, but also underwater. This is one of the more difficult constructions to build, as you tend to drown, you move slow underwater and it is dark. Furthermore, you properly want to drain the water out of your constructed building, while still allowing you to easily enter this. Luckily there are solutions to all those problems. For building underwater you properly want to use Gills Potion. If you are already further into the game you can find a Neptune's Shell, allowing you to breath permanently underwater, while also allowing you to move freely. To provide you with light you can use a Fairy Bell or similar items. To drain the water you only need a single Lava Bucket. Build a small enclosed space at the bottom of your house, remove the water inside with empty buckets and build some furniture inside. After that pour the bucket of lava in it and let the water flow in. The furniture will prevent Obsidian from being generated, causing all the water to disappear. To allow entrance, without buildings being flooded make an entrance from below using Platforms.
 * Sky Tower: Build a tower reaching all the way into space. Make sure to use different themes at different heights for a great look.
 * Museum: There are many items to display in Terraria. For example you can make rooms to display Statues, Paintings, Trophies, Armors, Banners, Butterflies, other Critters, and Weapons (on Weapon Racks).
 * Pixel Art: An alternate way of decorating your world, which is very popular in servers. Using different colored blocks (and/or paints), you can make sprites inside your world. Try making your own character too! Gemspark blocks will work better, especially if you want your creation to be visible in the dark.

For The Next Game
You've defeated all the bosses, got the best equipment and built a fantastic base. But what to do next? There are many options to make future games more challenging.

Game Settings
The game itself offers several settings which can make things far more challenging:
 * Expert mode is an option for world creation. Enemies and bosses become tougher and smarter, with more health, new attacks and debuffs, and AI improvements.  Enemies now collect dropped coins and despawn with them, while your own respawning after death is slower.  As well, NPC presence no longer prevents enemy spawning.  Several other game mechanics also become more difficult:  Healing is slower, debuffs last longer, water in cold biomes inflicts the Chilled debuff, and several events become more difficult.  On the flip side, enemy loot increases sharply, and bosses drop Treasure bags with powerful and vanity items that are only available in Expert mode.
 * Difficulty mode is a option for character creation. The character's difficulty mode applies no matter what world they enter.  There are three options here:
 * Softcore mode is the default; softcore characters only drop some of their coins when they die.
 * Mediumcore mode makes death similar to Minecraft 's: When a mediumcore character dies, they drop their entire inventory at the point of death, including equipment, accessories, and ammo.
 * Hardcore mode resembles the Minecraft world option of the same name: When a hardcore character dies, they drop all carried items (as in mediumcore), and also become a Ghost.  In this state, they can fly and pass through walls, but not affect the world in any way.   In multiplayer, ghosts are invisible to other players, but can still use the "chat" system to talk with them.  When they exit the world, the character is permanently deleted;  while items in the world (including items dropped at death, stored in chests etc.) are unaffected, any items in "personal storage" (Piggy Bank, Safe, Defender's Forge) are lost forever.  Unlike in Minecraft, the world itself will remain, and another character can enter the world to collect and use any items left behind.

Multiplayer, PvP, and Maps

 * Another player can be far more dangerous than any game enemy. Join a server, or host one yourself, and compete against other experienced Terrarians.
 * You can play Terraria maps made by other players, or create your own adventure map and share it.
 * You can play games such as Capture the Gem, or straight up PvP battles. A good arena or challenge map can make this much more "interesting".

Personal Challenges
Sometimes the game is too easy and you want to add an additional challenge, setting extra rules for yourself. This sort of thing isn't enforced by the game, but only by your own determination and sense of honor. Some examples are listed here; note that some of these will make completing the game impossible, the challenge is to see just how far you can get. Where advanced items are needed at the start, another character might obtain them and drop them off for the challenge character.
 * Only use ranged, melee, or magic weapons (or even a specific subcategory of those weapons, such as only Yoyos). You can use your starter sword until you acquire your weapons of choice if needed.
 * Don't use any potions (possibly including healing/mana potions).
 * Don't use any NPCs.
 * Don't use any storage items (keep the important items and toss the rest).
 * Never increase your health past 100 (at least until Hardmode starts).
 * Play as the Merfolk, by permanently equipping a Neptune's Shell and Fish Bowl. How far can you get? (Hint: Use Gills Potions and Water Buckets.) (Origin)
 * Play as dark elves by never setting foot above ground until you defeat the Wall of Flesh (Start by crafting a torch, after that dig underground and never emerge again) (Origin)
 * Reversed Gravity: Use an inventory editor to obtain a Gravity Globe and 100 Gravitation Potions to permanently reverse the gravity. (Use the Gravitation Potions whenever you want to use the fifth accessory slot for some time.) Try to get as far as possible. (Origin)
 * Buckethead: Equip a Bucket at the start of the game and never unequip it. This will indeed ruin every set bonus. (Origin)
 * Cursed Man: Set spawn in Dungeon. If you are on a server with other players enable PvP.
 * Bomberman: Mine only using dynamite/bombs (only Hardmode).
 * No Armor Challenge: Use no armor (only Vanity or armor as vanity). Potions that effect armor value or modifiers/accessories that affect armor value are not allowed either. (Origin)
 * Fairy: Cleanse the Corruption and spread the Hallow. Settle and live only in the Hallow.
 * No Crafting: Never craft an item.

Mods
There are various Mods available that introduce new items and enemies to Terraria, or alter the game rules. It is generally a good idea to backup your world and characters before installing any mod, as trying to remove a mod afterwards can "break" your world or character files.