Here's my recommendation for the return leg, starting roughly from Sacramento:
CA20 from Central Valley to US101, north through the Redwoods. There a various state and national parks along the way with good camping.
US191 from Crescent City to Medford OR, OR62 to Crater Lake.
US97 north to Bend OR (there are various volcanic sights near here)
US20 across eastern OR.
Continue on US20 across Idaho. Or better yet head into the Idaho mountains a bit visiting towns like Stanley and Challis. There are hot springs along the way, and a gold mining museum between Stanley and Challis.
US20 to West Yellowstone, through the park, and east to Cody. OR US26 to Jackson, Grand Tetons, and then east to Thermopolis (hot springs). Either way take US16 across the Big Horn Mtns to I90.
I90 to Blackhills, or detour off it on US16 to Jewell Cave and Custer State Park.
