Sadly, there really isn't anything to do (especially during those times) in the area around IAH, and the airport is quite far north from the center of town.

You could ride the inter-terminal train around the airport, that might kill 1/2 an hour or so... there are five terminals, so you could try to hit each one. If you feel so inclined, you could get out and explore, but since IAH puts everything of interest behind security it would probably be really boring.

The PC in Terminal E is the biggest and best one, so if you'll be hanging out that's the one you should go for.