Minecraft hole ideas?

Hi, I want to build a "hole" in Minecraft, so dig a very large area at heights 10-15. But I don't know what to do with the area! But it should still be an open space. Do you have any ideas?

Large, medieval hall
Cave with green stuff (i.e. Grass on the floor, leaves on the walls and so on)
Mines of Moria

Wow thanks I will probably mine from Moria.

Build a fat TPA trap. Cover the floor with lava (2 blocks high) and build a 2 * 2 platform from stones floating freely in the room. You can stand on it and then teleport any Opfakiddies to you. Zack - they are already being grilled in the lava:-)

I'm not building this on such a server haha XD