My room-mate in my first year of university and I used to build a bunch of things in the same world. He built a massive underwater fortress complete with a bunch of traps and secrets. I'm still convinced he hid some dark things down there.

We used to grief each other by hiding TNT around each other's work when the other had left for class or to go to the cafeteria. He had a massive roller coaster tower that "mysteriously" blew up on him mid-ride.