The weekly flohmarkt (flea market) in Friedrichshain's Boxhagener Platz is a popular spot where locals meet up for shopping, drinking, or just a bit of fun each Sunday. While the Mauer Park flea market gets most of the tourists, this one is far less touristy. You're much more likely to find a good deal on used books, cool T-shirts (some by local designers, even!), music records, and knickknacks.
Because the market is relatively small, it's quite enjoyable to take a stroll through the stalls and then enjoy a picnic on the grassy area in the middle of the square. Many cafes and restaurants are situated on the streets bordering Boxhagener Platz so you won't be bored for things to do.
The market is open Sundays during daylight. On Saturdays in the same area you'll find a fresh food & produce market.