Personally I would fly to PDX (Portland), and head west toward the coast. Spend a week driving as far south as the OR/CA border, take in the Redwoods, then head NE toward Grants Pass, stopping off at Oregon Caves. Then maybe Historic Jacksonville and from Medford head toward Crater Lake. Continue east to US97 north. There is a nice logging museum at Collier State Park. Plus camping. About a hundred miles farther north is LaPine state park. From here you could visit the Lava Cast Forest, several caves and lava tubes. Continue north on 97 until you hit the Columbia river. Several dams, fish ladders and near the mouth of the gorge a lot of water falls some easy hikes, some more challanging. This will give you a nice loop. If you have a day or so to spare, from PDX it is not too far of a drive to Mt St Helens.
