Spend a week in the San Juan Islands of Washington. Mid-late May is just before tourist season, so you should have the place to yourself (OTOH, be prepared for rain).
Easy ferry service between the islands, hills are moderate, scenery is breath-taking, and (shhh, don't tell) the weather is much nicer in the San Juans than it is in Seattle. There are 4 islands with regular ferry service.
Lopez Island is the flattest and easiest to bike--maybe 1-2 days. I recommend hitting this first as a warm up. You can also rent kayaks in Fishermans Bay on Lopez which is nice.
Shaw Island is pretty small and I'd skip it.
San Juan Island is nice, but the busiest (although in May or even June it won't be bad). Once you get out of Friday Harbor, you're biking on country lanes with rolling hills. I really enjoy San Juan Island. Lots of sights to see, too. I'd recommend going counter-clockwise on the Island. Make sure you hit Roche Harbor, English Camp, Lime-Kiln State Park and American Camp. Really enjoyed the camping at Lakeside Resort (private campground). If you stay there, I *HIGHLY* recommend grabbing a nice dinner at Duck Soup Inn. It's a couple of miles from Lakeside--you can shower at the campground a have an easy down-hill ride to the restaurant--best food in the Islands. Lakeside also has a nice lodge if you want to relax and take a break from camping, soak in a jetted tub, or catch up on all that wine you've been meaning to drink.
Orcas Island is the largest island and well worth spending some time. Lots of folks love Deer Harbor--I didn't find it anything special, and I'd skip it. Spend your time in the middle and Western side of the Island. Nice climb to the top of Mt. Constitution, too--beautiful views into Canada, Bellingham, WA and the puget sound from there. Also, Orcas Island has a lot of amazing pottery shops. If you're into that kind of thing, I'd recommend going to Orcas Island Pottery and Crow Valley Pottery--you can buy at the store and they'll ship it for you. Nice souvenir waiting at home.
Once you've spend a week in the San Juan's, take the ferry to Port Townsend, WA and then head south (either on the Eastern shores of the Puget Sound or head West and go down the West side of the Olympic Penninsula) and continue on down the pacific coast. Also note: if you're flying to Seattle, a lot of folks start a tour of the San Juans from Seattle. Just take the ferry to Whidbey Island, ride north on Widbey Island to Anacortes and catch the ferry to the San Juans from there.