Hi Whitey,
You have picked a very good time to be in Oregon. September is one of the better months for the entire state (both inland and the coast).
As mentioned earlier, there are several possibilities on your drive up the Columbia Gorge heading to The Dallas. I wont mention them here. But when you leave there and head to Eugene, take the back road, Hwy 197 south. On the way be sure and take a side trip Hwy 26 to the Painted Hills unit of the John Day Fossil Beds National Monument, you will not be disappointed. Back to Hwy 97 heading south, turn east at Terrebonne and head to Smith Rock State Park, plan for 3 hours exploring. Be sure and take Hwy 242 across the Cascades, it should be open this time of year.
The best-kept secrets in Oregon are the wineries in the Eugene area. Some worth exploring and for taking with you to the coast. Kings Estate has some very nice grounds and Broadley has some very nice Pinot Noir.
From Eugene to the coast, you are in Florence, home to the sand dunes. The crowds will be gone during the week, so you will find some solitude at the many hiking opportunities. Early mornings or late afternoons will provide the shadows and depth needed to take advantage of the contours. Other places of interest might be Heceta Head Lighthouse, Seal Rock State Park, Beverly Beach SP (after a rain), Cape Kiwanda SP, Oswald West SP, Hug Point SP (my favorite), and of course Fort Stevens SP.
Driving north along Hwy 101 to Astoria should be a real treat. There are dozens of parks and waysides to explore, it will depend on how much time you have. Any one of them could be your favorite. The weather on the coast is usually best in the fall, but remember this is Oregon, some people spell it R A I N.
Hope this helps and enjoy your trip. These are just a few ideas, I am sure someone else would have many more areas to share. If in Portland for long, give me a call and we can go to the Japanese Garden to photograph, I am always looking for an excuse to go there.
Patrick