Fun indoor amusement park
My son (10 years old) likes to go to Joypolis in Odaiba.
This is an indoor amusement park with various attractions like fun rides, haunted house and cool entertainments. The amusement park is on the 3-5th floor of DECKS Tokyo Beach mall that includes shops, cafe and restaurants so if you get hungry you could easily go out and get some food (of course you could return again).
Also, since it’s indoors you don’t have to be bothered by rain or hot/cold weather :)
My son’s favourites were the “wild” attractions such as Wild Wing, Wild Jungle Brothers and Wild River. There’s also an arcade with lots of games kids could enjoy.
Joypolis:
http://tokyo-joypolis.com/language/english/attraction.html
DECKS Tokyo Beach:
http://www.odaiba-decks.com/index.php?mode=information-en
misa