What is a good Minecraft challenge?

Make a city and turn it into an obstacle course for parkour and race to see who will be first.

Here's a little challenge I though up:

Nuclear war is on the way, and you know it. You have 1 Minecraft day to gather material and make it below Y level 60 before the bombs fall. Make sure you take some saplings and some wool. (If you don't mind some commands, you can give yourself some grass blocks at a 1:4 ratio with dirt blocks you must throw out.)

You must live the next X amount of Minecraft days underground. If at any time you open a path into something that can see the sky (ie dig into the side of a ravine) you must kill command yourself and seal off the passageway *without* retrieving your gear (as it is all irradiated).

At the end of day Y you may return to the surface, but only at night and only in a full decontamination suit (gold armor). If the sun catches you outside you must kill command. Note however that while you can loot villages you find, villagers do not exist yet and can't be traded with.

Z days later life returns to the surface enough that you can trade with villagers again, and it is safe to work in the day with decontamination suit on. What kind of underground society have you made in the meantime?