I live in Oregon near Portland. If you are coming soon, welcome! What time of year are you going? Best weather is July 5 - late September. October can be 'iffy' weather, as can June. September is often an incredible time.
From Seattle, you can consider taking the Victoria Clipper from Seattle to Victoria: Clipper Vacations Travel Deals: Victoria Clipper Ferry, Hotels, Vacation Packages
. I wouldn't recommend attempting to stay just north of Seattle and drive in/out of Canada. The border crossing, typically the portion back into the US can be a long wait. It became much longer after 9/11. If you feel you must do that, there are good camping options near Bellingham, Washington that you can consider. (I used to live in Bellingham and made several trips in/out of BC over the years). Don't forget your passports! Much has been said about Seattle as far as activities. Obvious ones are: Seattle Center (EMP, Space Needle, etc.), Ride the Ducks (WWII amphibious vehicle), Columbia Center (view), Pikes Place Market, Blake Island, Safeco field Mariner's game, Waterfront, Ivar's (seafood), etc. Consider a walk-on ferry ride for the experience or a harbor tour. There is a Seattle City Pass available for various discounts.
I'm going to talk here more about lesser known items in Oregon. Travel Oregon | From Mt Hood to the Oregon Coast, Explore Oregon
Oregon state law is that the beach is considered public land. Resorts can't section off a part of the beach for private use. This means tons of state parks and beach access opportunities. Among my favorite camping spots that I frequent on the north & central coast are: Fort Stevens State Park (Astoria), Beverly Beach State Park (south of Lincoln City), Devil's Lake State Park (inside the Lincoln City limits), South Beach State park (south of Newport).
"Must see" sites on the North & Oregon Coast include:
Multiple lighthouses along the coast are accessible to the public.
Astoria: Columbia River Maritime Museum, Fort Stevens and the Peter Iredale shipwreck, Astor Column (awesome view), Fort Clatsop (Lewis & Clark winter camp), Astoria Riverfront Trolley. Catch a lunch of Fish & Chips from Bowpicker across from the museum (takeout only, not a fancy place at all but best tasting in the area). There are some incredible Victorian style homes in Astoria. You can drive on the beach from Fort Stevens to Gearhardt. I've even seen motorhomes do it, but I don't recommend that. Fort Stevens has an incredible Civil War reenactment over labor day weekend.
Seaside: Visit Seaside, the Oregon Coasts Favorite Vacation Destination!
is more of a tourist trap, but fun. The promenade and prom street are where most of the restaurants, souvenir shops, arcade etc. are. Remember no sales tax in Oregon. Some folks I know personally like the thousand trails RV park in Seaside.
Cannon Beach: Cannon Beach, Oregon - Cannon Beach Chamber of Commerce and Information Center | Cannon Beach, OR - Cannon Beach
has the most photographed monolith in Oregon - Haystack rock. This is typically the photo you see when you see images of the Oregon Coast. Many art studios, and such are in town. Irish Table is a great small restaurant. Get there early if you want a chance at a table. Mo's is a very casual seafood restaurant nearby that is in several towns along the Oregon coast. Most known for it's clam chowder. To be honest, it is "meh", not as good as I have had, but it is a tradition of the coast and right in Canon Beach. Canon Beach RV park is nearby and includes a shuttle. We typically stay there when visiting Canon Beach and Seaside; includes a shuttle across the highway into town. Several other state parks are nearby.
Continuing down the coast are several additional views. Nehalem Bay State Park several folks enjoy for camping (I've not stayed there yet). I agree that Tillamook Air Museum (former blimp hangar) is enjoyable and of course you have to get some fresh cheese curds or ice cream at the Tillamook Cheese factory. There are some good clamming beds near to Tillamook. A friend has caught a large salmon on the Trask river (driftboat) and I've done the same in Tillamook bay.
While we are on the topic of air museums, if you can afford a little time, I highly recommend the Evergreen Air & Space Museum Evergreen Aviation & Space Museum | Wings & Waves Waterpark | McMinnville Oregon
that is about 1 hour inland from the coast @ McMinnville. This houses the Spruce Goose, a number of WW2 & cold war era aircraft, Russian, missiles, SR71, etc. The 'wings and waves' waterpark (waterslide from a 747 on the roof) is a hoot. Over 300 wineries are in this area of Oregon. Willamette Valley Wineries
This valley is known globally for the high quality of its Pinot noir. Many have tasting rooms and expansive views. Nice meal nearby? Consider the Painted Lady. Welcome to The Painted Lady Restaurant / Newberg, OR
There are RV parks near the Air & Space Museum but I also like Champoeg State Park - the site where the Oregon Territory (Oregon, Washington, Idaho, part of Montana) became part of the US by 1 vote.
There are so many other good places to visit in Western Oregon and Southwest Washington if you continue inland to Portland, Mount Hood, Central High desert (Bend, Sisters, Crater Lake) and the Columbia Gorge, but you said "coast", so I've limited to only one diversion.
Back to the coast from the Spruce Goose on highway 18 you will arrive at Lincoln city on 101. If you head north, Neskowin offers some nice views with romantic sunsets @ proposal rock. Nearby is tasty fare & pizza at Hawk Creek Cafe.
Lincoln City: http://www.oregoncoast.org/
has several shops, factory outlets, etc. To the north of town is Roads End State Park, where there are good tide pools (at low tide) to view. It's a walk on the packed sand from the park continuing north to reach them. We've enjoyed paddle boarding on Devil's Lake, nearby beach access from the campground and the worlds shortest river, blowing glass at Jennifer Sears Glass Art Studio in Lincoln City, Oregon
and much more. Turn right off of 101 on 51st street (Taft district) to get a taste of "yesteryear" on the Oregon Coast.
Continue on to Depoe Bay where, depending on the time of year, you can ride a charter to catch fish, view wales, enjoy some kettle corn or whatever. Tidal Raves is a great eating spot with an incredible view (perhaps the best from a coastal restaurant in Oregon). Depoe Bay Restraurant Tidal Raves
Beverly Beach state park mentioned earlier is not far away. Be sure to stop at Devil's Punchbowl State Park and Otter rock south of town. There is a small Mo's stand nearby for lunch if desired.
Continuing south to Newport, Discover Newport
there are many places to enjoy, such as the Oregon Coast Acquarium, Yaquina Head Lighthouse, Hatfield Marine Science Center, among others. Lunch and a tour of Rogue Brewery can be fun. Rogue Ales & Spirits
South Beach State Park is nearby for camping.
Ok - enough for now...