Mike, you'll enjoy Devils Tower. Another tip: As you're driving along the I-90 freeway through eastern Wyoming, watch for Pronghorn in the fields alongside the freeway. They are plentiful, but kind of blend in to the grass. Sometimes they run in small groups across the fields. It may be "where the buffalo roam, and the deer and the antelope play," but those Pronghorn antelope are FAST little guys.
A personal comment, in my opinion: In the Rapid City area, if you're tempted to go see the Crazy Horse Monument, you may want to resist the urge. They will quickly take your money, but they can't show you much more than you can see from the road. It's a tourist money-grab, and has been in the same relative degree of excavation for about the last thirty years. Save your big money, and buy a postcard in town.
Dave