don't do the underground tour...
Hmm. I liked it and my visiting family liked it. It might not blow you away but it is pretty interesting.
Things to do in Seattle:
Seattle Center (space needle, EMP, SF museum, dancing water fountain)
Catch the little monorail from Seattle Center to Westlake Center.
Catch a movie at Cinerama and have Top Pot donut before or after.
Shopping around Westlake or up Fifth
Pike Place Market (upper levels with food, flowers, produce and such, lower levels with interesting shops)
Various markets around Pike Place area like the sanitary market, post alley stuff. Lots of hidden restaurants and such here.
Keep going down from the market and cross under the viaduct to see the water front. Hit Ye Olde Curiosity Shoppe, have fish and chips, ride ferris wheel.
Head up like Cherry and walk down first to Pioneer Square for restaurants, galleries and homeless people. Also where the Underground Tour is.
Near Pioneer Square you can go to tour Safeco Field.
Can hit Ballard, Queen Anne, Fremont, Capitol Hill, Greenlake for various neighborhoods.
U-district to see U of WA, has some interesting architecture and in the spring cherry blossoms.
Overall though Seattle is a great place to live and an ok place to visit. Only a couple touristy areas and lots of non touristy neighborhoods. Plus lots of outdoors stuff I didn't mention like Mt Ranier.