There are several different flea markets (Flohmarkt) around Berlin, but Mauerpark is next level. It happens every Sunday and is huge, like football pitch huge. It goes on and on. Also, there’s tons of food, a club (of course!), and a beer garden with imported sand.
Besides all this, there’s an old stone amphitheatre with hundreds of people enjoying karaoke. Some people are pros and some suck, but either way you need to register hours in advance.