My new world to explore the new features of Minecraft 1.8.1, on the seed "Professor layton". Found the seed on the official forums, looked interesting...and it sort of is. There's a big old Abandoned Mineshaft underground right under spawn, and a large underground Stronghold not too far away. There's supposed to be another stronghold and an NPC village fairly close, but I haven't found them yet. Mostly I've been spelunking so far, but I built one outdoor and one underground Chicken Coop, plus roofed in a bunch of chickens who trapped themselves in a shallow ravine. Most other stuff is underground.
Update: Well, explored that stronghold...and now I'm digging it up, archaeology-style! The area has been roofed off with an enormous 50x64 glass roof, and inside I'm busily excavating all the rock and soil above the Stronghold until I can expose the whole thing to sunlight. I recently went exploring a bit on the surface, and am wondering if anything else interesting may have turned up in the cave map.