Ditto on Leavenworth. Small, but very interesting Bavarian themed town. Used to be a lumber town that died, but the locals came up with the theme idea to attract tourists. Hwy 2 over Stevens Pass is fun and interesting - lots of twists and turns. :)
I'd avoid I-90. Many miles of grasslands and not a lot to see unless that's what you like. It can be VERY windy at times and even semi truck drivers have to pull over until it dies down enough. Even blows over FW trailers.
If you don't mind driving up to Mt. Vernon, take the ferry over to Port Townsend. Great ferry ride and Port Townsend is a quaint town with lots of of old buildings. From there you could take 101 down towards Olympia and back around up into Seattle. Or take 104/3/16 to Tacoma and back up to Seattle.
Definitely gotta visit Pike Place Market and the surrounding Old Town. Another must-visit is Ivar's on the waterfront across from the Market and get some of their famous clam chowder.
A bit south and east of Seattle is Mt. Ranier which is a good visit. Or go to Mt. St. Helens a bit more south. You'd need to go to the south side of Mt. St. Helens to see where the eruption took place, but it's a few more miles south. We visited the north side of Mt. St. Helens this year and is worth the visit if time/distance restricted.
If you've come this far, maybe you'd want to go right out to the coast like say Ocean Shores. There's a number of good campgrounds there too. By Sept. it starts to get quiet out there as the summer folk have headed back home.
Be prepared for some horrendous traffic in and around Seattle. Leave your RV at the campground! Parking around Pike Place Market can be a beetch and you'd be better off with the Harley...
If heading to Wa. in the next month or two, check up on active forest fires. It's been a bad year up and down the west coast. We were in Leavenworth last August and there was a forest fire burning nearby. All the locals were in panic/high alert mode in case the winds shifted and the fire got out of control. It has been hotter than normal and very dry out here for several months. Washington has a statewide drought and the southwest portion of BC is in a record level 4 drought condition (the worst level).