If you have small children of varying age and live in Seattle, the Children's Museum ( http://www.thechildrensmuseum.org ) is a good place for a membership. One adult and one child costs $15, and a year membership is as low as $40. So, go three times and you break even.
If it is raining outside, we often go here so the kids can get exercise. There is plenty of space to run around and it's a good place for a play date.
Our favorites include:
1) The art studio where the kids can do arts and crafts. Each time we go, there is a different project to make. I'm not sure if it's the parents that like it more or the kids. We like to paint here!!!
2) About two times a year, there is a new traveling exhibit. Clifford is the current exhibit.
3) Discovery Bay: for Kids 3 and under, the water table is a popular destination. But it takes second billing to feeding the fish.