I would tend to discourage trips into the mountains in May - the drive will be long, and May can be very iffy - if it's raining or gray in the lowlands (as is often the case) then that usually means foggy, rainy and/or snowy in the mountains, depending on elevation. Combined with a long drive (and a boring one to Mt. St. Helens) I don't think the mountains are that good a bet.

You might consider Snoqualmie Falls in May - they will be extremely impressive with all the water coming off the mountains as the snow melts. The falls are about 45 min. by car from central Seattle. http://www.snoqualmiefalls.com/

I'd also strongly second the idea of doing the Mukilteo - Whidbey Island loop. Even if you don't want to tour the Boeing plant - www.futureofflight.org - you'll drive by it - biggest building in the world. Plan to stop at the very cute waterfront town of Langley on Whidbey Island, also Coupeville. If you want, you could take the Coupeville - Port Townsend ferry, visit PT, then return to Seattle via the Bainbridge Island ferry.

Or else you could keep going all the way up Whidbey to Deception Pass - a very impressive narrow channel between Whidbey and Fidalgo islands. You return to the "mainland" via a bridge, near which is the town of La Conner, another very attractive little settlement. This is the area where all the tulips are grown; there might be some stragglers visible when you're there, depending on when in May. It's about an hour to return to Seattle via I-5 from the Skagit Valley.

Another island option is to head to West Seattle (exit from I-5 south of downtown) and take the ferry from Fauntleroy to Vashon Island. Vashon is a lovely rural island, with a very picturesque little town at Quartermaster Harbor halfway down. From the south end of the island you can take the ferry over to Tacoma, then return to the freeway (and airport) perhaps with a stop at the Museum of Glass - a great little facility for those interested in art glass. Dale Chihuly's Bridge of Glass is right above it - well worth a stroll. http://www.museumofglass.org/