Hi midon1, "water and nature" we can do :-)
Since you'll be stopping at Mt St Helens, you might want to spend the night around Astoria. Although it's not on the ocean, it's an interesting old town. There is an excellent maritime museum, the fun little trolley and you can climb all the steps to the top of the Astoria Column which is well worth it for the view on a clear day.
If you are interested in Lewis & Clark, there is a good interpretive center at Cape Disappointment in SW Washington, about 40 minutes from Astoria.
Okay, so that night plus Seattle plus Portland on the end accounts for 8 nights and I think you said in your PM that you have a total of 16? So that leaves about 8 nights "extra" for your trip.
Do you want to spend all of that time on the coast? Or perhaps see some high desert or mountains? Crater Lake? Redwoods in California?
What are your thoughts on what you'd like to see and do?