I went here on a day trip from Yogyakarta, only visiting the main museum called Museum Manusia Purba Sangiran.

The museum was ok, nothing great, and I would probably be rating this much lower had I already been familiar with the subject matter, but it was cool to go in somewhat blindly and see the skulls, learn about the excavations, etc. for the first time. There are about 3-4 different rooms/halls some of which (particularly the 1st and last) are very focused on homo erectus as well as other animal remains, fossils, etc. I seem to remember the 2nd room being a bit of a mess, my memory fails me a bit but there were a lot of random displays about anything from climate change to Immanuel Kant in here. We also spoke to some international students who were studying anthropology (or something similar) and working in the area after finishing the museum, and they mentioned how some of the displays seem to imply that Javanese people today are direct descendants of homo erectus, which is obviously false. Also, as someone else has pointed out, it was difficult to tell in some rooms whether the skulls were real or not due to lack of signage, etc. Hopefully they can give the rest of the museum as good of an overhaul as the 1st room seemed to have.

As for getting to and from the site, there is a train from Yogya which goes to Solo, a town nearby the museum, and along the way stops off at Prambanan, so it made sense to me to do these 2 UNESCO sites in one day. But it's a bit of a fuss getting there. Once you are at Solo, it is quite easy to get a bike (via Gojek, Grab, etc.) to the main museum, and it's quite a nice scenic (but bumpy) ride. But it was seemingly impossible to get a bike back to Solo from the museum area however, and we asked a few locals who eventually showed us to a bus station from which a minibus took us back to Solo.