This weekend is Opening Day of the Seattle boating season. Even if you don't own a boat, you can enjoy the warmer weather and rent a canoe and paddle around the Arboretum. (Don't do it on Opening Day, though.)
Here's a nice map courtesy of the Northwest Outdoor Center. Pay heed to avoid the green section marked Caution. I made a wrong turn and ended up in the Caution area, and I found myself slogging through very shallow water.
Assuming you avoid the shallows, it's a very pleasant experience. Commune with the ducks and other birds. Pull over at one of the beaches and take a lunch break (assuming you remembered to pack lunch). Wave at the cars stuck on the 520 bridge.
Bonus reading material: The Miracle of Foster Island.