Perfect time to come. I don't think there is a more beautiful State than Washington in May when all the Roddies are out in bloom, but then I'm a little prejudice. Also you should be able to see migrating shore birds by the millions, and whales also.
Coming up 101, make sure you check out the Redwoods as there is nothing so majestic as seeing those trees. Stay at Jedediah Smith State Park if you can. In Oregon, pick one or two of their State Parks cause you just can hardly go wrong with any of them and make sure you visit some of the lighthouses. Don't pass Tillamook without stopping for an ice cream cone,(you will see the Tillamook Cheese building with the tons of cars and RV parked around it.) Spend some time in Astoria and get a history lesson about Lewis and Clark.
Camping at Cape Disappointment will not disappoint .Also coming up the coast, stop at Ruby Beach, its just a day use pull out but you won't be disappointed at all. If you don't have a camera, your phone will do.
If you have a passport or enhanced drivers licence, walking on the ferry to Victoria (in Port Angeles) just for a day trip is worth both the time and the money. If you can spend a few days there, you will not need a car at all, as most is in walking distance of the pier. Dungenous Spit is a nice place to streatch your legs.
Port Townsend is also a must see, the town and Fort Warden are both good bets and if you can get reservations at Fort Warden all the better.
Happy travels.