The Berliners are thrifty folk and their flea markets are awesome, brimming with everything from antique toys and art deco jewelry to boxes full of secondhand kitchen goods and clothes of prior decades. Though websites online offer curated lists of Flohmarkt, or flea markets (here, and here, and here), some of the info might be out of date, so always go prepared to ask for locations and times of the markets should you show up at a location and find it empty.
Here is the Berliner Trödelmarkt, which had an excellent selection of both cheap and antique jewelry and toys:
And yes, the flea markets happen even when it rains like crazy, like it did when we visited the Mauerpark Flohmarkt. This market had way more handmade goods, clothes, and boxes of odds and ends: