Asobōno!

Fun for littler kids

If you have younger children and you're in the Bunkyo area of Tokyo, Asobōno! is a great place to check out. It's an indoor play center which is perfect for days when it's either too hot or too cold for outside play, and there are a range of different themed areas, including a "sea" area (complete with a giant ball put area) and a forest area. The standard admission fee covers an hour of play, with additional fees applicable for every 30 minutes thereafter. I'd say it's best for the 5 and under crowd, kids older than that age might find it a bit dull.