Any visit to Seattle that doesn't include even a short ferry trip is one that left out a real experience. Because of its geography, Seattle relies on the State ferry system to provide access to local islands and other communities.

A fun short ride (35 minutes one way) from the Coleman Dock, the main ferry terminal, would be to Bainbridge Island. Once off the ferry, stop for coffee or lunch, see the galleries and shops, with a nice walk through Winslow, and then return to Seattle.  It only takes a few hours to experience beauty, peace, fresh air, and the uniqueness of western  Washington.  

The Bremerton ferry is twice the time and cruises through narrow passages, and you can see a few galleries in Bremerton, navy ships, and more.

If  you have a full day, consider the San Juan islands ferries. The trip and island views are scenic. Washington state ferries run out to four of the San Juan islands (San Juan, Orcas, Lopez and Shaw) and depart  from the mainland only in Anacortes.  Expected wait times for boarding these car/passanger ferries is normally about an hour, athough in the summer it is wise to allow for a 2 hour wait to board especially on Fridays and the weekend.

San Juan and Orcas Islands are ideal for a longer visit..  See Inside Page, below.

For a day-trip ferry jaunt, San Juan Island, with the town of Friday Harbor and its many shops and galleries, is a winner.

http://www.tripadvisor.com/Travel-g58...

The Seattle Water Taxi offers a quick and easy way to travel across the harbor to West Seattle. Departing from pier 50 (just south of Coleman Dock), the 15 minute ride offers great views of downtown Seattle. Once in West Seattle, two free shuttle lines transport passengers to the shops and restaurants of sandy Alkai Beach or the West Seattle Junction. For taxi times, fare pricing, and other information, see link below:

http://kingcounty.gov/transportation/...

Another short ferry ride will take you to Whidbey Island.  See inside Page, below.

http://www.tripadvisor.com/Travel-g58...

Important reading follows, especially if headed to the San Juan Islands:

http://www.wsdot.wa.gov/ferries/