Head over to the Ballard Locks right now to watch coho and sockeye salmon milling about in open water outside the fish ladder, and crowding behind the glass in the underwater viewing area. They're big, big boys, and there are a lot of them.
You can also watch cool boats and ships navigate the locks; let the kids run down the steep, grassy terraces on both sides of the locks; climb trees; and pose for silly pictures by the octopus tentacle sculptures (can't miss those).
It's all free, except for the few bucks you'll shell out for parking (bring change).