Here are links to the events you can visit during your stay in Zagreb in September:
Free Spirit Walking Tour
https://freespirittours.eu/tours/free-walking-tour-zagreb/ (individual booking only).
Projekt Ilica: Q’ART
https://projektilica.com/en/about/ (kind of a bloc party on the main street Ilica on September 18th, arts and crafts, music, food and drinks).
Artupunktura – Zagreb art therapy
https://www.artupunktura.hr/en/ (throughout September).