Walk on as a passenger on one of Seattle's ferries. Rain or shine, the views of the city skyline, water, and mountains (or clouds/fog) are sure to be spectacular and the ride relaxing.
The Seattle to Bainbridge ferry is 30 minutes one way and $8.75 for a roundtrip adult walk-on passenger ticket.
If you have time, walk a short distance from the ferry terminal into downtown Bainbridge for coffee or a scoop of ice cream and catch a later ferry back to Seattle. If you'd rather get back to the city, step off the ferry and get right back in line for the return trip.