Oh, mostly the forest-y and nature-y states - Arkansas, Colorado, New Mexico, Utah, Idaho, Tennessee, Kentucky, Arizona, Washington, Oregon, California...I would've liked to see more of Montana and Michigan, but it gets cold up there in the winters.

Mostly I drove through Texas to avoid Oklahoma (one of my least favorite states), but the Guadalupe Mountains were very nice.

I'm not sure I'd call it 'clean' though; the air seemed awfully smoggy and I saw almost as many flaring wells as I did in North Dakota. But there were lots of open spaces and the people were friendly.

Washington, Arkansas, New Mexico, and Utah get my vote for the best places to do a parks/forests vacation, depending on your interests.