Rent a canoe at the University of Washington Waterfront Activities Center and paddle around beautiful Lake Washington. It's a great place to see some interesting wildlife, and if the weather is clear you'll also get mountain views.
Canoes fit up to 3 people and are $14/hour for the public. Catch the Light Rail in downtown Seattle and ride to the University of Washington stop -- you'll pop out just near the stadiums and the Waterfront Activities Center. Paid parking is also available.