I build a tower/lighthouse that was 70 units high constructed as such:
(top down view)
you have to jump up and to the right over a gap for every step. I wanted to get to the top to plant a tree and it took me a day and a night to successfully jump my way to the top.
Had a cemetary. Named each dead play-through based on their deeds during thier life. Even buried a 2x1 wood plank "coffin" for each one. The signs said things like "THE FOUNDER. Killed in the mines by a creeper". "THE ARCHITECT. Fell from Dragons Tooth Tower", "THE MINER. Sacrificed himself to save the castle walls", and so on and so forth.
When I found my first dungeon, I had no idea about the mob spawners (back when these were new and we were able to pick them up), and there were a bunch of skeletons that were spewing out after I made a 1x2 hole in the wall of the dungeon. After a few tries of trying to get a door to seal that hole, I got it sealed and put a sign that said:
Wish I still had it, but when I read up about being able to pick up the mob spawners (and finding out about the "Peaceful Mode" (seriously, I had no idea)), I went in, picked it up, and removed the sign.