We have been out on a few trips to the National Paks of the US
Have you seen the national park website?
http://www.nps.gov/index.htm [nofollow] search on each park and you will find lots of info on the walking.
We did Grand canyon Bryce and death valley in one trip, but went to yellowstone on a seperate trip. I think you would find its a long drive between them
There are all amazing places and well worth going. Like anywhere you don't have to walk far to get away from the crowds - I'm sure I read some statistic once that said that 90% of visitors to Americas NP's never left the tarmacsurfaces!
A few tips from our experience:
If you can stay in the national park itself then walking early morning or evening can give you better weather and less crowds (especially places like the Grand Canyon. The NP campsites are in fantastic locations but are usually fairy basic
You don't need a pass for day walks, but often do for anything longer, eg overnighters. Walking down and staying at the bottom of the Grand Canyon gets booked up at least 12 months in advance.
It gets very hot in Grand canyon and death valley. We we went in spring and that was hot enough for us. had the surreal experience of 40 degrees at death valley one morning then skiing at mammoth mountain in the afternoon! Downside of spring is that Tuollome (?spelling) meadows road at Yosemite will still be closed.