If you're looking to have fun on one of the lakes around Seattle, the Electric Boat Company is a great way to go about it. They rent the neatest electric boats, which are different from your standard boat with an outboard motor. They're positioned on Lake Union, and rental rules require you to stay in that body of water; so, no traversing over to Lake Washington or out into the Sound.
Think of an electric golf cart, but with seating for 12 people (no kidding, I've done it). The boats are anywhere from 20 to 24 feet long, and are similar to a pontoon. The boats are equipped with bench seating, a center table, and canopies. Everything is removable, so they can be used rain or shine. They're set to run at a maximum of 7 knots, which is the no wake limit in certain parts of Lake Union.
They rent by the hour for two hours at a minimum. You can bring on your own coolers and food, or you can stop by any of the restaurants on the water (think Ivar's on the north end of Lake Union). We liked them so much, we bought a season pass. The boats have heaters on them, so we can use them year-round.
They're a family-run operation and are great people to deal with. I highly recommend them.