Welcome aboard the RoadTrip America Forums!
There's simply no way to have a bad trip on your route, particularly for a photographer. Some time back I gathered up some of the
best recommendations for this general neck of the woods. On a more general note for your trip, I would try to alternate between the coast and the Cascades both southbound and northbound. If you were a little more pressed for time, I would suggest taking the coast one way and the mountains the other. But since you can, mix them up. This makes for a more varied and enjoyable trip and increases your chances of getting good weather for at least a portion of each scenery type. The other thing I think you'd really like is a drive up the old Columbia River Gorge Highway. There are about a dozen waterfalls just a short hike from this road as well as the vistas of the Columbia River itself.
AZBuck