Of all the tourist things I have ever done, The Seattle Undergound tour is one of the best values ever. Hilarious and well-priced. Almost as good as The Cosmos of the Black Hills.
I like spending time on the water. Definitely a ferry across to Victoria or one of the other islands. There isn’t a heck of a lot to do, but I would do it just for the boat-ride. The kids will like driving the car onto the boat.
There is a national park across the way, though it is one of the less interesting ones.
Go see a Seattle Sounders game if the kids are into soccer at all. It is the loudest, most exuberant soccer environment in the US.
Eat crab and Salmon.
Pikes Place market is a must-see. Something for everyone there. I like the place with the tiny little doughnut maker, if they are still there. I could watch it for hours.
I think they have a children’s museum there, or a science center or something like that. We were in a rush so we popped in and out but I think it was pretty cool.