Ideal: one-day or weekend trips; out-of-the way locations; little auto traffic.
I’m in Seattle and here are a couple of favorites:
– San Juan Island, the county seat of Island County. The island is mostly flat terrain, hilly only around the old limestone kilns in the NW corner. Easily done in a day. Highlights: British and American camps (from the Pig War); False Bay – U Washington research area that empties at minus tides; town of Friday Harbor has good restaurants; and B&Bs throughout the island. About 35 miles around the island.
– Lopez Island, which has less traffic than San Juan and is more rural. Several pocket parks and a wonderful state park at Spencer Spit. Take a picnic lunch, as there’s only Lopez Village to buy supplies. The state park is likely to have bike-in camping even on the busiest of weekends. Also about 35 miles to circuit the island, but lots of ways to cut the trip shorter.
– Mukilteo-Deception Pass on Whidbey Island. Cross to Clinton on the ferry, then stay off WA525 as much as possible, following the county roads through Langley and Freeland, then up the west side on Bush Point Road to Fort Casey. Fort Casey is a wide-open former coastal defense fort with 7" guns mounted on the old platforms. Through Coupeville and onto the Kettle bike trail. This trip is about 65 miles each way.
None of these are rigorous. They have vertical rises of less than 150’ and follow mostly flat terrain. Some resources for those interested:
Bicycling the Backroads Around Puget Sound, Woods
The San Juan Islands, Mueller