From time to time, museums in Germany have an open night, a so-called "Lange Nacht" (long night) or "Museumsnacht" (link goes to list of museum nights in Germany and beyond). In addition to museums, galleries often open their doors during those events, and there's usually some extra programme and special events for the visitors. Plus it's usually a pay-once-visit-all-of-them event, for a ridiculously low price. I have been to those nights a few times, partly as a visitor and partly as a participant, and it's always been a load of fun, so I can very much recommend those events.
And there's one coming up: Mainz will have a "Lange Nacht" on June 16, and the full programme is due to come out on June 4. The entrance fee is already set at 10 Euros, which is about nothing for the possibility to visit 52 participating institutions - and even though the official programme is not out yet, I can tell you that the RGZM will participate and also offer special programme - including my tent with demonstrations of spinning and tablet weaving. And I'm already looking forward very much to being in that lovely venue again!