What is your planned route?
If you're doing Yellowstone, don't forget Grand Teton Nat'l Park which is right next door. They are completely different in looks and things to do.
We always stay IN the national parks for the best experience.
All national parks have a web site. They give lots of good information - campgrounds, things to do, road updates, maps, etc.
Your route could go in a circular form so you can take in different things on your return. For instance, the southern Utah national parks are awesome - Zion, Bryce, Capitol Reef, Canyonlands, Arches. Then work down to the Grand Canyon in Arizona. Depends how much time you have. You could easily spend the whole summer!