A brief list of fun things to do in Seattle:
The Underground Walking Tour in Pioneer Square is a humorous primer on early Seattle History, complete with traipsing through dank underground passageways.
Every visitor to Seattle must make a stop at Pike Street Market. Check out the vendors and watch the flying fish.
For a bit of peace, head to the Olympic Sculpture Park, a free branch of the Seattle Art Museum (also a good place to go).
For views on a clear day, you could go to the top of the Space Needle, OR you could save your $$ and head up to Kerry Park on Queen Anne Hill. Alternately, you can head to the top of the Columbia Tower, the tallest building in Washington State--significantly cheaper and taller than the Space Needle.
Like some other big cities, Seattle has it's own version of a duck tour. Ride the Ducks leaves from right across from Seattle Center. After you get your fill of cheesy jokes on your amphibious truck tour, you can head across and check out the Experience Music Project/Science Fiction Museum--one ticket gets you into both venues.
For beach fun, head to Alki Point in West Seattle (take the Water Taxi there from Downtown Seattle!) or Golden Gardens Park in the Ballard neighborhood.