Everything that has already been posted and add Newberry National Volcanic Monument near Bend, OR. Don't know anywhere else you can drive to the top of a volcanic cinder cone. Nearby Mt Batchelor has some great scenery as well.
Now if you run out of things to see in Oregon come on up to Washington. Mount St Helens Volcanic Monument is pretty cool stuff. Mt Rainier National Park has spectacular scenery but it becomes one big traffic jam pretty early on summer days. North Cascade National Park has some awesome scenery and a lot fewer visitors. Then there's the whole Puget Sound to visit and did I mention the San Juan Islands?
You'll want to start planning your next trip right away...