Need a fun, free activity for the kids on a sunny day? I highly recommend the Ballard Locks! My kids love to watch the boats going through the locks and the water rising and falling to accomodate them. I think it's pretty neat too.
Be sure to explore the park on one side and the salmon ladder on the other. (Now is a great time to see the salmon running.) There is an observation window under the water, so you can watch the salmon leaping upsteam from below - pretty cool!