There are a ton of interesting things to do "around" the Seattle area. We generally stay in 2 separate locations to take best advantage and spend 4-5 days in each. We just day trip into Seattle with the toad.
If you stay in Port Townsend, WA you can stay at Fort Worden State Park.
Fort Worden is an old WWII era coastal artillery defense fort. It is located right on the Straits of San Juan and the campground is a real treat. You need to reserve early for summer sites and the lower (beach) campground is the way to go. The kids (and adults
) can crawl all around the old gun emplacements, bunkers, etc. Plus it's very convenient to Port Townsend's downtown area. PT is full of great restaurants and is an old victorian seaport with some really neat architecture. Be sure to stop at The Elevated ice cream shop - it's a classic. It's also a great base camp for day tripping in Olympic Nat'l Park's Hurricane Ridge area and Sol Duc Hot Springs. If you head out to nearby Sequim, you can drive through the Olympic Game Farm, which is the old Beebee ranch where Lloyd Beebee did all the filming for Walt Disney's animal movies. You can feed and pet bison, elk, llamas, etc. You can even throw bread to grizzly bears (but you can't pet them unless you want to be called "lefty" for the rest of your life
The second location is Anacortes, WA. We stay at Fidalgo Bay RV Resort
when here. Again, ask for the lower campground which is right on the bay. Anacortes is another great place. It's here where you can take the ferry to San Juan Island for a great day trip, take an excellent whale watching cruise (Orcas), and day trip to Seattle. Last time we arrived in Anacortes we took the North Cascades Nat'l Park road from east to west. After 5 days in Anacortes we drove down Whidbey Island and put the Bus ion a ferry from Keystone to Port Townsend. After spending another 5 days in Port Townsend we drove around the Olympic Peninsula, stopping off at the Hoh Rain Forest, and then boondocked right on the Pacific Ocean by a state campground called South Beach, just south of Kalaloch, for $8 a night and hiked on the beach watching bald eagles and sunsets in the evening.
If time is tighter, just take I-5 south from Port Townsend but be sure to stop off at Mount St. Helens. Take the west entrance in and take the RV all the way to the Johnson Ridge Visitor Center at the end. You won't believe how amazing the view is. I climbed it last summer and looked into the crater from the rim but it "burped" last October so I don't think they will be allowing climbing this summer because the lava dome is rebuilding.
As far as Seattle itself, I personally can't spend more than a day there because there's way too much stuff to do "around" it, than "in" it. We always stop at Pike Place Market
(bring a cooler to load up on seafood deals). We've seen the Space Needle but unless you want to spend $50 for a family of 4 just to take the elevator I can't see it. A 3/4 mile walk down from Pike Place takes you to Pioneer Square, where you can take the Seattle Underground Tour
and walk underneath the streets of Seattle and hear about the early Seattle culture - it's very interesting and the tour guides turn it into a real comedy show and make it very entertaining. We skipped the Boeing tour but our friends took it. It is a big building and is impressive in it's size but we spent that day taking the ferry to San Juan Island where we saw the American Camp and English Camp NPS sites from the "pig war", visited Friday Harbor (Free Willy was filmed here), watched bald eagles at Lime Kiln Point State Park, and visited the Whale Museum there.
There's lots to do and not enough time to see it all. I hope some of these ideas will help you decide what you are most interested in.