It's all good and hard to recommend!
We were in Jasper & Banff last month. Very interesting and very mountainous (burned out the engine in our F250 on the way home). If you want to see lots of wildlife, you want to go to Jasper. Didn't see any in Banff. The national park campgrounds are awesome. Nothing like seeing Moose wander mere feet from your campsite.
We're just near Mt. Hood. Love the area. Close enough to visit Portland. Timberline Lodge is fascinating. Not too far to Bend which is very different terrain in high country. Many places to camp around Mt. Hood, FHU or dry camping. Mt. St. Helens is worth a visit (south entrance).
Can never go wrong with the Or. coast but don't know what's available in the way of dry camping. The Mt. Ranier area is interesting and farther north, Leavenworth is an interesting visit. It can start to get cold in Sept. up in high country east of the mountains from OR to BC/Alberta. Few years ago in Bend, the entire CG unexpectedly froze up overnight in early Sept.
One thing to be aware of in Aug/Sept, is the potential for bad wildfires. Later Aug. & Sept. can be the worst times. Travelling closer to the coast could be a safer bet. Last Aug. we ended up towing our TT alongside a wildfire that had just broken out near Peachland BC. Few seasons ago we bailed out at the last minute on a visit to Yellowstone in early Sept. due to a large wildfire nearby.