The Gorge will probably be crowded, especially the waterfalls that are essentially road-side but they are still worth going to.
If you want to aim for fewer crowds, Eagle Creek along the Gorge takes you to Punchbowl Falls which is awesome, there's another waterfall farther up on the right for which there is a distant view, then you hit tunnel falls where you walk under an old tunnel blasted through the mountain and past that there is one more.
The gorge trail to all those is spectacular in the fall and I think its probably about 4-6 miles each way to see them all. That can be a full day.
There is another one for which I can't remember the name that may be a little less crowded. Its on the road that goes past Multnomah all the way at the end of it going away from Portland.
Any tourist info will have a map of the more popular ones along the gorge. There is usually a waterfall section in the magazine type things at rest stops around Portland along the Gorge.
I don't remember how far it is from Portland but Silver Falls state park has the Trail of Ten falls. Should be spectacular this time of year. 10 waterfalls on one trail and probably 7 of 10 are spectacular.
All kinds of other stuff in that area too.