I just built a spiral staircase tower that has hit the game ceiling. Having a lot of fun with this game and I haven't even really started exploring yet, just building a home base cave+tower near my spawn point at the moment.
Here's a picture of the work so far:
The top is above the clouds. It's pretty neat to watch the sun come up from the top. I'm building it with dirt and glass, hoping to make all the stairs have grass on them. Still have some work to do cleaning up the undersides of the spirals so that they are uniform and there's no wasted blocks. I'm going to put torches on the top so that it can act as a homing beacon when I'm out exploring.
Also, everyone knows that you can jump and put a block underneath you, right? If you have a bow & arrow you can jump-block up higher than even the skeletons can shoot arrows (can't remember the height, just play around with it), then fire down on the mob(s) that are trying to attack you with relative safety.
edit: Here's what it looks like after I've tidied it up a bit:
I'm calling it the divetower, because there is water at the bottom, and when I want to get down I just jump.