We went to an amusement park with the Nanzan Judo Club. It's part of a larger resort, but we were only there for the roller coasters, man.
It's 4100 for the ticket if you want to ride all the rides (のりもの放題） as much as you want. There's a cheaper ticket that let's you in, but you have to pay for rides separately.
As far as amusement parks it's pretty similar to all the others you see around the world. What's different is the haunted house they have open all year round (as far as I know). The monsters are different from what you'd see in the US (to an extent - The Ring has made creepy little girls with long hair a recognizable horror), and that much is definitely interesting. Otherwise, it's the same as any other amusement park you may have visited - PURE FUN.