I have to confess, I haven't been up the Space Needle. However, the City Center all around it is great, especiallly over Labor Day Weekend when they host a four day festival called Bumbershoot.
Pike Place Market is good and fun for tourists and locals alike. The Seattle Art Museum is good.
If you have music fans in the group, go online and get advance tickets for the Experience Music Project. If you have a rental care, you can also visit Jimi Hendrix's grave in Renton, WA just south of Seattle.
Remember this is the home of Starbucks. Also, SoDo MoJo stands for South Dome Mojo and is a reference to the Seattle Mariner's baseball team.
If you have time to take the ferry to Bainbridge Island, that is a great day trip.