I recommend a few indoor activities, for those who don't want to stay out too long.
- Aquaria like the Shinagawa aquarium, Sunshine city aquarium and Osaka aquarium are quite good. In summer, they also have the occasional event where the kids can get some water sprayed on them.
- Museums are quite good to see in summer.
- Amusement parks like Tokyo Disneyland have both indoor and outdoor attractions.
- Water parks like Shimizu Park are good, too.