First of all almost anything underground is great, cold, dark, dank, yeah, lots of bad stuff is going to happen...Also I like keeps and castles, but mostly keeps and I have been designing tower adventures. It is kinda cool to have pc's run through a 4 to 8 story tower, each floor is a suprise and it is easy, as the DM to set up the encounter.
Outdoors, I like because of the randomness I can use to build an adventure, you can run into almost anything in the great outdoors!!....What say you??
thanks