There are a lot of galleries in Auguststrasse, which is just a short walk north of the Hackescher Markt S-Bahn station. Here is some information on it: https://www.visitberlin.de/en/arts-auguststrasse
There are several modern art museums in Berlin, but unfortunately a few of them are closed for renovations. Two that are still open are the Hamburger Bahnhof and the Berlinische Galerie. I'd encourage you to check out their websites to see if any of the special exhibitions are of interest.
In terms of public art, the most famous is probably the "molecule man" who stands in the Spree River. There is some more information here: https://berlinstreetart.com/public-art-berlin/ I haven't taken that company's tour, but it might be of interest. Have fun!