Around St Helens check out the Lower Lewis River Falls Campground. It's dry camping but super nice paved spots and usually water is available. I say "usually" because one of the two pumps has been out each of the last two years I've been there and last year both were down the day I left.
Anyway, the water falls are AMAZING and there are great day hikes from the campground. It's a bit remote but worth it. Distance wise you're about 2 hours outside of Portland or a little more than 1 hour outside of Hood River and beautiful Columbia River Gorge.