The Space Needle
Pike Place Market. Ivars, the Aquarium, and Ye Olde Curiosity Shop.
She's hit the high points you could do the second line all in one day. Add the foot ferry to west seattle for something different and you can get lunch at a fish and chip shop on the other side.
http://en.wikipedia.org/wiki/West_Seattle_Water_Taxi I second the Space Needle as a good place to get a unique view of the city.
Some other not "must sees" but depending on the interests of your party good things to do:
http://www.tillicumvillage.com/ It's interesting if you're into native american culture and the food is good. Only thing is that it's gotten pretty expensive and you're stuck out there on the island based on their timetable. But I've taken most of the extended family and the hae claimed to enjoy it.
http://www.undergroundtour.com/ So FUN! and amusing and interesting from a historical standpoint. Seattle once burned to the ground, when it did, they sluiced the hills behind and built the roads above the ground floor. There's a whole underground city under there. The tours are humorous, not much to see, but still fascinating and interesting to be under the city above. Again, multiple guests have said they enjoyed it.
http://www.museumofflight.org/ Seattle is about logging and airplanes - the museum of flight at Boeing field is really a good one if you're into that sort of thing.
http://seattletimes.nwsource.com/news/local/library/ The downtown central library is a major tourist attraction these days. Pretty neat looking and a nice respite if you want to chill during a day of walking the city.
http://www.seattle.gov/tour/locks.htm The Ballard Locks are fun at certain times of the year when the Salmon are spawning. Also if you have a kiddo into trains and boats and mechanical things it's pretty fun to watch the boats move in and out of the locks. Ballard itself is really interesting - it's where the Scandanavians who moved to Seattle settled originally and there are still signs that they are there - swedish pancakes and the like. It's gotten more "hip" lately though. They have a nice farmer's market in the summer on the weekend.
The university of washington campus is beautiful too. In short, you really can't go wrong in Seattle! Enjoy your trip and post pictures!
Urs