I originally posted this in the "General Terraria Discussion" thread, but there was hardly any response before it got buried, so I figured I'd take my own advice and give it its own topic so more people would see it.

The original post: Though, for the purposes of this thread, ANY kind of self-imposed challenge you've attempted -- or considered attempting -- in the game is up for discussion. =)

Here are some ideas I can think of. Note that I have not played the game, so some might not be feasable.

Pacifist: You must not harm any enemies. (If an NPC or pet harms somthing, this doesn't violate the challenge.)

Pursuer: You must kill every enemy that spawns. If an enemy despawns, this challenge has been failed. (Not that this includes bosses and events.)

Poverty: You may not keep rare items or materials, and must not keep more than a certain amount of money. You may not keep chests or other containers. Common materials and drops are allowed to be kept, and you may use those to craft items.

First night challenge: Choose a boss, and you must kill it before the second day.

No NPCs challenge: You must not allow any NPCs to take up residence. The most obvious way to do this is to not build buildings suitable for housing.

Low health challenge: You may not use anything that would increase your maximum health.

One biome: Choose a biome and make that biome take over the entire world (or at least every area that biome could possibly go).

True Rogue: Play a Hardcore character, and you must stay in one world, which must not exist prior to creating the character. (Try combining this with other challenges.)

A couple more ideas:

Prohibition: Don't drink any potions.

Not so hard: Defeat all Hardmode bosses using only pre-Hardmode equipment. (Also, no using Hardmode equipment even outside of boss fights.)

Here's one, don't start over every ten minutes because you don't like the look of your base foundation. :P

I dare anybody to use the axe and pickax only. Must be pretty hardcore to try and defeat tWoF with a pickax. Probably not achievable, but respect if anyone tries :P

Some interesting ideas here, I suppose, but has no one actually TRIED any such things? Y'know, "I'm bored. I'm gonna try to [verb] the [noun] without [verb]ing." Surely I and Unkalibriert can't be the only ones! (SPOILER: The self-imposed challenge he did was a teensy bit more impressive than mine.)

People have done speedruns of this game. Typically, the goal is to kill a certain boss, but some categories invovle events or things like killing all pre-hardmode bosses or getting all NPCs.