The Hiram M. Chittenden Locks carry boats between Puget Sound and the fresh water of Lake Union across a 22-foot difference, using nothing but gravity, and they have done it since 1917. Watching a lock cycle is free and genuinely absorbing. The better half is underneath: a fish ladder with a glass viewing gallery where you can stand below the waterline and watch sockeye and chinook climb through on their way to spawn, strongest from June through September. The Carl S. English Jr. Botanical Garden is on the same site.