Skills, again.

Okay, I have passed skills interface.
Then, I expanded world under the soil and added one lair. It eats up about 250mb. =)

So, you would get your dungeons.
There would be big underground world, you can enter dungeon, find small path and enter big tunnels systems.
Find underground vault, that doesn't has access from surface, towns of antmen etc etc
Potentially, you can stay in Underworld as long as you want.
Wait, wait, wait. At the beginning, it would be just dungeons.

EDIT: Nearly forget. Next step, some basic character development. So, player would have some reason to move around.