On a trip back from Nebraska I didn't have time to stop anywhere in Iowa, (or anywhere else, really) which sort of sucked.

But if you're driving straight through and don't expect to see anything, I vote against I-80 in Iowa. It's very flat, which means if you have, say, a rainstorm crossing you from northwest to southeast, it tries to blow you off the road, and you're fighting the wheel... periodically you'll pass a small knob off to the side of the road, and suddenly you're fighting against nothing.

I never went off the road, but it was very hard driving, and I was going the rest of the way back to PA that day. (I drove all save from Steubenville to a few miles from my house myself)