Well it's vacation season, so be prepared for Winnebagos and crowds. What to recommend depends a lot on what kind of stuff you like.

If you are starting or ending in Vegas and driving all the way, a trip through Death Valley would be an easy hit; I like deserts. Agree w Tony that the volcanoes of Cal-Or-Wa are great if if you've never seen then, but I am also hugely partial to the coastal route, especially the less developed section between Crescent City, CA and Coos Bay, OR.

Whether you are up for day hikes or just want to take in the view from the parking lot, it's pretty easy to make a loop around Mt. St Helens or the Olympics (you can score some pretty fantastic scenery by driving up to trailheads like Deer Park or the popular Hurricane Ridge)

Lastly, FWIW, I've heard mixed reports on the Dairy Queen at Donner Pass....

Jim