THe Hiram M. Chittenden Locks (also known as the Ballard Locks) is situated half in Ballard, half in Magnolia, and is connected via the locks which have walkways connecting the two sides.
The fish ladder is on the Magnolia side, and the large parkgrounds are on the Ballard side.
Go when the fish are running which is generally between June and September. You'll see salmon so big they just make your mouth water (that is, if you're a fan of Salmon, which I am not).
If you're lucky, you can see some otters or sea lions outside of the locks grabbing their share of tasty swimming salmon. You'll also see the salmon outside the locks as they try to get in and out.
This is a very large and beautiful park and the best part is THE ENTIRE THING IS FREE!!! How many great things like this are free of charge? Not many, that's for sure!
If you go during the summer, parking can be tight on the Ballard Side. Cross the ballard bridge, go into Magnolia, follow some windy roads and park on the Magnolia side. Traffic is much lighter and the parking is actually closer to the action than on the Ballard Side.
Bring your camera, pack a lunch, and spend one of the few sunny SEattle days out in the open watching fish do things that they've been doing for millions of years