The Mercer Slough Nature Park is a great place for quick casual kayaking, canoeing, or fishing, located at the northwest side of where major highways I-90 and I-405 meet (yet quietly tucked away from the hubbub). The park encompasses over 320 acres of fresh water wetlands and is a rare mixture of wildlife habitat, active agriculture, and office buildings all sharing the same marshy area.
There is a good boat launch that is very gradual and right by car parking for easy loading in and out. From there, go left and paddle through trees, lily pads, and an active blueberry farm. We saw many ducks and turtles sunning themselves, and apparently beaver and nutria are sometimes around as well. Then the water forks and you can go either way in a loop around an island that has semi-camouflaged office buildings on it, all up on pilings since they are built in a floodplain. On the other side of the island there is a short spur that ends at the culvert outflow from beneath I-405, feeding the wetlands with new water. If you were to go right at the initial boat launch, that direction would soon take you out to Lake Washington.