Day Trippin' 101
Here's a tasting of the smorgasbord of day trips accessible from Seattle. It is just a sampling, with ideas for a variety of settings. Each of these outings is a full day except for Snoqualmie Falls which can be a 1/2 day excursion.
Mount Rainier is the largest mountain in the state and is a gorgeous national park. It is about a two-hour drive from Seattle.
Skagit Valley & the town of La Conner are about an hour north of Seattle. The valley is one of the richest areas of farmland in the country and all kinds of flowers are grown there (although most will be past their bloom season in late May). The town of La Conner is a farming and fishing town that is also very pretty. There is interesting shopping and good food there. The Museum of Northwest Art, in La Conner, is a great showcase of the northwest style of contemporary art and craft: .
Mt. Pilchuck is just east of the Skagit Valley, with great hikes in the park: http://www.localhikes.com/Hikes/MountPilchuck_7602.asp and Pilchuck Glass School.
Whidbey Island, is about 1 1/2 hour away, including a great 1/2 hour ferry ride across Puget sound. The town of Langley is very picturesque and Ebey's Landing is a great walk both over and along a beautiful beach.
(Above: The Ebey Landing walk)
Snoqualmie Falls and Salish lodge are about 45 minutes east of Seattle, toward the Cascade Mountains, in farm country. The falls are quite dramatic. The lounge in the lodge offers a fabulous view of the falls and surrounding countryside.
Tacoma is about an hour south of Seattle. Things to see there include: The Museum of Glass, founded by Dale Chihuly, the Tacoma Art Museum (which I think has a better, albeit smaller, collection than the Seattle Art Museum), and Point Defiance Park which has a great 5-mile loop drive as well as many trails and great views of, and access to the beaches on, Puget Sound.
Although this list just scratches the surface, it is a great place to start.