I spent a week at Rushmore and I think a couple of days there would have been more than enough. I agree that Custer State Park is well worth the time. My favorite there were the wild burros.
I spent a month at Yellowstone and saw something different every time I travelled through the park. I plan on going back and doing it again. My favorite part of the park was the Lamar Valley in the North East. We saw more varieties of wildlife there than any other part of the park. I saw badgers, black bears, elk, bison, wolves, and pronghorns. I went back just about every day and saw something new each time.
The Buffalo Bill Cody museum in Cody, Wyoming is definitely worth the time if you like guns. I spent two full days just going through there.
We planned a week at Teton, but we could have spent a couple of days and seen everything. The big attraction there is the scenery.
