I have ridden around the Oylmpic Peninsula, mostly on the main highwayt 101. It was a great ride of 322 miles. It will probably be quite wet Fri-Sat, especially on the western parts.
You can also ride from Anacortes down Whidbey Island, another rain shadow area. On Whidbey, you can take a ferry across to Port Townsend. and then work your way back to Seattle by, for instance Bainbridge Island and Windslow ferry. Check the maps, ask locals, etc. This is all great biking country, when not too wet.