If Cedar Point is on your list, a trip either up through Michigan (or down, depending on entry point) will take you through some spectacular scenery. Sleeping Bear Nat'l Lakeshore offers beautiful camping/hiking/biking; the Lelenau (Michigan's "pinky") area is scenic wine country with quaint little towns; Ludington State Park offers beach access on Lake Michigan which should still be relatively warm in Sept - and the drive through the Upper Peninsula is reminiscent of being in Alaska: beautiful, wild, remote.
Spetember on the west coast of the USA and Canada can be smokey due to forest fires - there's a website called "inciweb" I think, that shows where the fires are (and places to avoid if you enjoy seeing lovely vistas)
If you decide to nix Cedar Point - Ed and I followed the Pacific Ocean from N Cal to Oregon (if we had a month, we would have gone all the way to Canada), and that was a spectacular drive (and upwind from any fires)