I used to live in seattle with my ex and my oldest daugter- she was two at the time, but we still had a ton of fun. Kids love Pikes Place Market- and it's free. You can spend a whole day there, it actually takes up four floors which most visitors don't know, and leads down to the waterfront.
Take cash- in fairly small demominations- have free samples for lunch, and maybe a fresh sausage (there is a stand that has something like 50 varieties, you can buy by the pound, or just get one and have them grill it with onions and peppers to eat like a hotdog). Get chowder from Ivars (the pierside stand or on the ferry, not the fancy restaraunt) and watch the guys from the Flying Fish work- if you decide to buy some seafood they can pack it in a container that will keep it fresh for 48 hours to bring back on the plane. The waterfront has a hoard of other activites as well- an old deisel sub that you can go on (I did it VERY preggers, that was a tight squeeze), an aquarium (fairly decently priced if I recall correctly), you can watch the fishermen on the peir catching dinner for the expensive-restaraunt-goers
About half an hour north of Seattle is Everett- Port for the USS Abraham Lincoln, try to find out if it will be in when you are going, and if they are offering tours while you are up there- a real joy for little boys of all ages (even the grown up ones) There is also a berry farm just north of Everett- if you pick your own they are dirt cheap, and that can be fun too. Blackberries will be in season for another couple of weeks, but you can pick buckets-full out of any empty lot in the area.
Another short drive away is Boeing's headquarters, and I know they give tours, which I beleive are free.
Bruce and Brandon Lee are buried in the area, I can't remember precicely which cemetary, but if you go, their graves are by the flag pole at the top of the hill.
Across the sound from Seattle is Bremerton (a ferry ride away, walk on if you can, don't drive if you don't need to) were the Navy has their dry docks for the entire west coast, there is sure be at least one masive ship lifted out of the water- the propellers on the aircraft carriers are the size of a small house. Again, see if you can find a tour, if not you can usually see them fairly well from off base. There is also a war plane museum in the area.
I have to leave for work, but if you need more ideas, send me a message.