In 2005 when I lived in Japan I made sure to visit Puroland aka Sanrio's indoor theme park. Now when they say theme park it's a little bit of a stretch as they only have one real ride. But hey it's one heck of a ride...a boat ride that takes you past all the cute sanrio characters! If you have any love for Sanrio you'll love it. I'd recommend Puroland just to check out Hello Kitty's Pink house. Be warned when I went I was the only person over 5 that wasn't accompanying a kid. It's definitely aimed at a young demographic :P
Cute statues as you walk towards the magic rainbow
The Boat Ride!
Hello Kitty's House!
Make sure you get a photo with Hello Kitty Herself!
Links: Official Website (English)
If you're in Japan why not go to the Hello Kitty Cafe too!