I'm so thankful I took the trips west and visited those goregous national parks years ago.  Today, I would not be able to endure the crowds at most of them and tolerate all those vehicles of every description clogging the roads to and from, in and out of the parks.  It is a trip everyone should take at least once in their lifetime though.  Early to mid-May and in September are good visit times--schools are in session everywhere and the throngs are much smaller.  In the higher elevations you'll find snow piled twice as high as your vehicle along the roadside but the roads are clear and you'll go back & forth from very warm temperatures to very cold.  I have a photo of myself building a snowman on top of Pikes Peak in late May.

Yellowstone, Yosemite, Bryce, Zion, Rocky Mountain--is this where they are or were digging out a dinosaur in a mountainside and there's a museum there too?  Mainly in Utah's,  Wyoming's and CA's parks.  Unfortunately, I never made it to the Grand Canyon but did get to B. C./Banf and Lake Louise in Canada. Also did the New England, Nova Scotia and other Canadian provinces route by auto.  I never made it across the pond, however.