Here's a fun game to play when in a strange building with time to kill.

With some friends wander off in a direction that looks interesting. When you come to a door that looks like you aren't really supposed to go through, try it out. When you see stairs leading down from the main level, take a look at what's down there. Just try and explore as much of the building as you can in the time you have, and see what you can find. Personally I like learning the layouts of big, interesting buildings, and just plain old exploring.

Of course, urban spelunking requires some Common Sense(TM). Here's tips:

So, have fun, and happy spelunking!