Roofed Forest Superflat Challenge
Challenge: Get as many achievements as possible.
Although I haven't verified this myself, it should be possible to get all the achievements in the game. There is lava and water available so obsidian can be made. You can get iron and diamonds from mineshafts and dungeons, although you will need to search for both as they spawn with normal chance. There are also resources around the lava lakes and sand around the water.
Be aware though that mobs spawn all the time due to the darkness under the roofed forest. The lava does produce forest fires which helps thin out the forest a bit in areas. This should be a fun challenge even for the hardened Minecraft player.