Flea Market
I went yesterday to a giant flea market that happens every Sunday at Mauerpark. Here you can buy clothes, food, bikes, CDs and records, furniture and every random thing. I just browsed around and got some food.
There is also an open mic karaoke that happens at one part of the park, but I missed it. It's supposed to be very fun and the crowd that shows up to watch the singers, both good and bad, are very supportive and cheer every singer on.
Something I learned from a local was that the city had sold a part of this park to a private investor who intends on building luxury apartments in a few years. Berliners are upset and worry that the flea market and the whole neighborhood will change once the development starts.