Yellowstone will be crowded so if you want to stay in the park, you should make reservations ASAP. Fishing Bridge is centrally located but YS is huge and traffic can be....errrr.. a bear. ;)
Cody, WY is a great town to visit. Eat at Irma's, see a rodeo, take in Buffalo Bill Museum and make a day trip to Red Lodge, MT. From Red Lodge, take the Beartooth Hwy west to Chief Joseph Hwy. Beartooth has been rated as one of the most scenic highways in the USA and we agree. Absolutely breathtaking!
You might double check your route from Cody to Devil's Tower. Going over the Big Horns can be a challenge. You can drop south out of Cody to Thermopolis to Casper & to Gillette then to Devil's Tower if you want. Beautiful drive through Wind River Canyon on RT 20.
After Rushmore and the Black Hills, there is a lot... Wall Drug, Badlands NP, Minuteman Missile, Pioneer Auto Museum (worth the stop) at Murdo, SD & the Corn Palace at Mitchell is worth a look.
Have fun!
