I wrote a bunch of challenges on my iPhone one day, thought I would never use them
-Upside Down (use the "flip" shader and voila!)
-1 Type of Food (only eat one specific type of food)
-No Sprinting
-Lefty Mouse (use your mouse in your non-dominant hand)
-No Chest
-1 Chest
-No-Crits (never go for crits)
-Combo (after using an item once, you must use a different item next, e.g. rod once, get a hit, a FnS, 1 bow shot, rod once, hit, ect.)
-Sword Switch (switch from two different swords between hits)
-F5
-F1
-F8
-Rainbow Shader!!!!
-30 FOV
-Bad Hotkeys (if you have a numpad on your keyboard, change your controls so you can only switch items using that pad)
-Invis Chests (use a texture pack that makes all chests transparent!!!!)