Are you arriving soon? I ask because an afternoon in May is quite nicer than an afternoon in August when the heat is at its worst.

I would definitely leave the airport - 8 hours is a long time at IAH. If you go online and look at some of the options you might see something of interest. my thoughts would be perhaps -
A visit to NASA's rocket park - for me - seeing the old Mercury rocket (Apollo rocket) is worth a visit.
The Battleship Texas isn't too far - perhaps an acquired taste but better than sitting in the airport for 8 hours.
You could rent a car and just go for a drive - if you've never been to Houston.
Some people who have never fired a gun might enjoy a shooting range.
At the very least you should go out and get some great food - BBQ - Tex Mex - steaks...