This list is not complete.
Space Needle (Competed Jan 4, 2014)
Aquarium (Completed Dec 28, 2013)
Woodland Park Zoo
Watch Salmon run
Go Whale Watching
Discovery Park (Completed Feb 22, 2014)
Gas Works Park
Botanical Gardens (there are two. One is in Seattle and one is in Bellevue)
Seattle Art Museum
Olympic Art Garden
Native American museum
Asian Art Museum
Pioneer Square gallery
A show at one of the early venues Nirvana played at
Kurt Cobain pilgrimage
Visit Jimi Hendrix's grave
Visit the first Starbucks.
Major sporting event
Wander around Fremont and find: the troll, Lenin, the rocket, and other street art.
Visit Mt. Rainer, the Olympic Mountains, and the North Cascades. (not in the same trip though).
There are some things I'd like to add: like find the good restaurant, the main road race for the city and participate, and some of the other pop culture stuff, find a really awesome coffee place, and shop the more interesting boutiques. These ones are more nebulous but when I figure out what the are I'd like to do them.