I'd recommend taking the interstate through South Dakota to see Wall Drug and Mt. Rushmore if you haven't been there, and then detouring to Devil's Tower in the NE corner of Wyoming.

If time allows, perhaps a longer detour north of Missoula to the National Bison Range is in order? You could then head to Quinn's Hot Springs. Otherwise, keep on the freeway and visit the Montana Valley Book Store in Alberton. In any case, don't miss the 50,000 Silver Dollar Bar.

See the center of the universe in Wallace, Idaho. Then the Museum of Arts and Culture in Spokane.

Head to the Toppenish Northern Pacific Railway museum, then go south to see the Maryhill Museum.

A drive through the gorge is next, and you can see waterfalls on the Oregon side, or Beacon Rock on the Washington side.

Then, in Portland, Oregon, there is a zoo and the Oregon Museum of Science and Industry.

If you head to McMinnville, the Evergreen Aviationuseum is home to the Spruce Goose and more.

Finally, head to Tallamook to see the cheese factory, Newport for the Oregon Coast Aquarium, and plenty of lighthouse museums in that general region if you're still interested in those at all.