The Underground Tour, The Museum, Old Town. You should consider buying the City Pass. We have bought them and definitely found them to be worth it. Here's what's included in the price:
Adult $79
Child $59
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Your CityPASS Includes
1 Space Needle
2 Seattle Aquarium
3 Argosy Cruises Harbor Tour
4 Museum of Pop Culture (MoPOP) or Woodland Park Zoo
5 Chihuly Garden and Glass or Pacific Science Center
and here is the website:
http://www.citypass.com/seattle#ym5Y0mf29xFd3qVK.99. Have fun. Don't forget to check out the restaurants and the original Starbucks at Pikes.