I'm considering renting a 4wd vehicle. Obviously, it's not a lifted rubicon but any unpaved high altitude trails in Oregon or washington that may require brief, safe, legal use of low range?
Only in winter where there are sections that require chains or 4wd.
We did Portland and Seattle this past February. Had a rental Mini Cooper that was a lot more fun to drive than any rental I've had before and great for getting around the cities.
Portland itself you can spend days just wandering around on foot enjoying the place. Food is awesome. Pok Pok is a must as is VooDoo donuts. Deschutes Brewery makes some good beers. Have to go to the vegan strip club, Casa Diablo. Portland is the strip club capital after all.
Checking out the Goonies sites is fun if you're into that. Hit up Astoria to see the house then out to Cannon Beach to see haystack rock. A bunch of sea lions and seals hanging out there in Astoria.
Seattle - Go to the EMP museum. Awesome museum of music and sci-fi/fantasy stuff. They were running a Nirvana and Hendrix exhibit there when we went. Pay to go up the space needle only if it's a clear day where you can see the mountains. Definitely go to the market. Skip the aquarium there it's rather mediocre for being on the ocean.
The big draw of the area of course is nature and the parks. Trek out to Crater Lake. See the painted hills. Go romp in the forest. Go watch Mt. St. Helens smoke. Etc
Moving out there later this year. The place is awesome.