A few places you may not have stopped before:
Big Horn County, Wyoming, east of Cody - Pryor Mt Wild Horse Range, Bighorn mountain range, etc. worth seeing
Northeast Wyoming - Devils Tower
southern Montana - Big Horn National Battlefield - worth a day or 1/2 day visit
Dillon can be a good base for visiting some state parks and historic sites
Idaho - I second those who recommended Craters of the Moon and the Sawtooths. No decent CGs with hookups in the Sawtooths IMO, but some wonderful NFS CGs for dry camping along Stanley Lake and Redfish Lake. Arco KOA is a good base for Craters, or use the park's campground (no shade, no hookups & it can be HOT in the summer, but interesting scenery).