I'll be flying into Portland on a Sunday in late September, renting a car and driving to Hood for a several-day vacation. I'd appreciate any suggestions you might have about places to visit/things to do while I'm there. Thanks in advance.

Mt. Hood? Hood River?
Almost doesn't matter, as they're all so close it's like one and the same.
My suggestion: Do a search here and on your favorite search engine. There have been many threads on this exact question lately - you'll find all sorts of answers to your questions have already been posted - you just have to bother to look.

Check out either July or August Sunset magazine. There's an article on Hood River(the town). I'd also suggest a drive through the Hood River Valley. It's beautiful.

BTW-I'd skip meals at Timberline Lodge.

If you check out 10 Days in Oregon under text you will find wonderful suggestions that were given to me on my recent trip to Oregon. We enjoyed a wonderful lunch at the Multonah lodge inn; a not so wonderful lunch at Timberline. The hikes were great along the Gorge and unless you are into windsufing or similar activities, Hood River may not be the place for you.

Go to your local Barnes & Nobel and read what Frommer has to say in his Oregon book. We find his suggestions for Oregon to be very helpful.

There is an older historic "highway" from Portland to the Hood River area that goes by numerous waterfalls. It's a great drive.
You can then stop at the Bonneville Dam to see the salmon and other fish going upstream from a glass window in the visitors building.
We stopped in Hood River at the Holstein Coffee House. Definitely a locals hangout, with good beverages.