Hanayashiki in the Asakusa area of Tokyo has the distinction of being the oldest amusement park in Japan (and also has Japan's oldest roller coaster). Area-wise, it is extremely small, but there is a ton of stuff packed into that small area. The roller coaster circles the perimeter and there are lots of small family-oriented rides in the center. You can either pay a small admission fee and then buy tickets for the rides, or you can buy a free pass which gives you unlimited rides. We just bought tickets and did a couple of rides that we wanted since it was two adults, but with a family, I could see spending a few hours here riding everything for a very relaxed and fun afternoon that I think kids would really love without the crowds and miles of walking in bigger parks. I highly recommend the "Surprising House." It was definitely different from any attraction I have ever been on. Odd...but lots of fun. There is also a stage where, while we were there, they were having some type of Halloween show, but since it was all in Japanese, I'm not really sure what it was about. All-in-all, this is a very charming little park that is well worth a visit while visiting the Asakusa area. You can just pop in, buy tickets and ride whatever you see you like, and not take up too much time out of your day unless you want to spend more.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC