We went a couple of years ago, flew into Sea-Tac and stayed in Portland for a couple of weeks.
Cannon Beach is lovely, but the Pacific coast can be cold and windy, even when it's hot and sunny in Portland, because of the range of mountains in between, so don't expect beach weather. Don't bother with the Tillamook cheese factory (dull, although the ice cream is good)
The Columbia river gorge is worth a drive, maybe hike up to some of the waterfalls. Mt St Helens is interesting. Mt Hood has the hotel where The Shining was filmed, and a fantastic donut shop on the drive up. Plenty of history - Lewis & Clark etc.
Portland itself is a great little city, but we were staying with friends who took as to their favourite places to eat and shop. The DC enjoyed the zoo and the science museum.
2-3 days in Seattle was enough. You can buy a visitors pass which gets you into most of the main attractions (space needle, museums, boat trip, aquarium and, possibly, Boeing). We also took in a baseball game and a soccer match, went to the original Starbucks and the markets. Stayed at the HI-Express near the Space Needle, which was fine for the price.
Re flights - we flew BA, and hired the car through Avis, their preferred partner. We got a good rate, and were able to pick it up in Portland, dropped it at Sea-Tac.