I spent the weekend up in Seattle a while back and really wanted to visit a few places I hadn't been in years. One of those places was the Ballard Locks. Boats are constantly passing through between Puget Sound and Lake Union so there is a lot see. Also certain times of the year salmon are running through the fish ladder and you can actually watch them as they swim through.
The 'Salmon Waves' sculpture by Paul Sorey was one of my favorite sites (photo 3) at the Locks.