Visit the Ballard Locks! During the summer, there's always activity there. You'll see trawlers - and everything else! It's especially entertaining on weekends, and around Seafair and the hydro races on Lake Washington. Plus, the locks have beautiful gardens, a fish ladder, etc. They are one of Seatle's most popular tourist attractions. We were through the locks recently, around the Fourth. I've been transiting the locks for 30 years - always an adventure.
Phil's suggestion re Fishermen's Terminal is a good one - no gates on the piers there. Good places for lunch and dinner right there too - Chinook's is a great seafood restaurant, and the Longliner Tavern is a fave hangout for the fishermen who're still around.