<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Is there much to see in Yellowstone?
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Ummmm, 70+ miles of very cool (hot!) geothermal stuff?
I believe you could spend a month in Yellowstone and not see it all. I regret only having two or three days the one time I was there. The most wildlife I've seen in any of the parks, bubbling mud, hot springs geysers and steam everywhere, nice lake, cool geological formations, and the biggest log cabin structure in the world.
Mt Rushmore was a bit of a disapointment. Definitely worth seeing if you're close, but the view from the visitor center makes it seem much smaller than pictures that you see. It's a bit like Niagara Falls. Spend five or ten minutes looking at it and you're done. Then, you realize you're in South Dakota, which is full of wonderful people, but offers only Wall Drug and the Corn Palace as other options for sight seeing (not meant as a slam, just trying to point out how wonderful Yellowstone is compared to Rushmore).
For my money, I'd spend the time in Yellowstone. I wonder if Denver is your best place to fly into, but then again, you should have a beautiful drive up!
JP