suggestion on cross america

I am thinking about driving across America sometime next month. The whole trip will be around 3 weeks and I will leave from Philadelphia to California and then back to Philly. I want to cover all the major scenes along the route, so far I can only think of Yellow Stone and Grand Canyeon. One of my friend suggest the desert in Arizona is also great. The route is flexible but I get to finish it in 3 weeks, +/- a couple of days. Any suggestions of where I should go? I like the beauty of nature, no so much fond of big cities. Thanks.