You can find information about free programs in your city or region on the Internet by entering the keywords Free offers for children and your city or state into the search engine.
Offers in Brandenburg:
http://www.masgf.brandenburg.de/cms/detail.php/bb1.c.216537.de
https://www.reiseland-brandenburg.de/ausfluege-tourentipps/
https://www.mamilade.de/brandenburg/alle/ausflugstipps/schlechtwetter