I think that would be a very long day. You might be better off to get a hotel in Portland for the night, and then spend the next day or so exploring there. Personally, I don't think there's much worth seeing I've been a few times, but with specific destinations in mind (meetings mainly, though I did make a trip specifically for donuts that was overall not worth the journey). I believe it's popular with foodies these days, so if you're into that sort of thing maybe some folks can make some suggestions on "must eats".

I've done this drive a lot. I think pretty much the only thing worth seeing is Mt. St. Helens.

Totally viable, and definitely the best option. The drive between Portland and Seattle is boring enough, you wouldn't want to do it twice in a week.