Ballard Locks
One of the most visited attractions in all of Seattle is also one of the most utilized pieces of infrastructure in the area, the Ballard Locks!
More officially known as the Hiram M Chittenden Locks, this complex of locks at the west end of Salmon Bay is operated by the US Army Corps of Engineers. The waterway can take on boats as large as 760 feet and as small as kayaks. It was completed in 1917 and connects the Puget Sound with Lake Union and Lake Washington. Once in the lock, the water level drops 26 feet and freshwater is exchanged for saltwater as the ships make their way to the Puget Sound. The whole process takes about 10-15 minutes, depending on how long the line to get into the locks is.
There is also a fish ladder here, something you can view during the summer when salmon and steelheads are running. There are 21 steps or 'weirs' that make up this ladder, and you can learn more about it, the fish that swim through, and the locks in general at the onsite Visitor Center. There are an attached gift shop and several nearby restaurants as well, such as the Lockspot Cafe.
If you have extra time be sure to visit the Carl S English Jr Botanical Garden before you go!