Devil's Tower is a great stop. Our family loved it more than Rushmore.
The SD Black Hills are gorgeous too.
My wife and I just came through there last month on our way home from an eight week trip. We were coming the other way back from Minnesota. We drove through North Dakota and then came south to get to the Black Hills. What a gorgeous drive.
Wyoming is amazing, but she's a tough lover. (Two of our kids went to college at the University in Laramie, so we've spent plenty of time there.) Don't underestimate the distances and the weather. We got caught on a windy day (even by WY standards) and the wind somehow cut through one of our slide-outs and blew out the bedroom window on the opposite side. I've never seen anything like that before. And that wasn't the first time Wyoming took a swing at us. The rest of the trip was crazy cold with just plastic duct-taped across where the window should have been.
Everyplace in WY is awe inspiring except for the drive across the 80 in the south. The 80 is bleak and windy. Avoid it if you can. The rest of the state is mind blowing.
Your time frame and distances are very reasonable. You'll love your trip.