A floating sphere shaped arena filled with stairs, platforms, a lava 'engine' and a dark area at the top for potential monster spawning.
I think its 50x50 blocks in size, something like that. No items so you can hide chests full of goodies for players to find if you want. Also it will fit in a nromal sized world so you don't have to clear a hole to bedrock to fit it in.
The sphere was made with MCEdit while the rest was made by hand.