We visited the newest themepark in Indonesia, just opened in June.
It is located in Sibolangit, about 20 minutes before Berastagi on the road Medan - Berastagi, so about 1 hour from Medan. It is at a slightly lower altitude than Berastagi, but much cooler than Medan (though still hot in the midday Sun).
There are some bumper cars (aka dodgems), a rollercoaster (which was quite uncomfortable, and not exactly inspiring confidence when I saw them push the cars around the last flat bit of the track on the previous running, and also because you have to worry a lot about Indonesian maintenance standards on such a fast ride), a big ferris wheel, a flat train, a water inner tube slide (not finished when we were there), about 7 or so other fairground type rides such as a tree house, then in the indoor area (a nicely designed fairy tale castle) there is a very good dimsum restaurant and some arcade machines such as the basketball game.
The cost is 12,000rp to enter, then 10,000 or 12,000rp per ride.
There were only six cars in the carpark when we were there, and a lot of the ticket booths (inside the park) were closed, and so the KFC and most of the restaurants were closed due to lack of custom. It's apparently busy at weekends. All the rides still operate, you just walk up and give your tickets and they will turn it on for you, even if you are alone.
Although it's quite a nice park for Medan with some decent landscaping, it's not a patch on what you'll find in the West. If you are visiting on a short break to Indonesia it's probably a waste of your time as you'll find far better at home, though if you are taking local family/friends they will be impressed, and as it's just on the road to Berastagi, if you are visiting Berastagi, you can potentially only stay half-an-hour before moving on, so it needn't form a big part of your holiday.