All great suggestions so far. I also VERY HIGHLY recommend Nil Imbiss, a Sudanese lunch joint. Their standard thing is a spiced "falafel" served in pita with yogurt, eggplant sauce, peanut sauce, lettuce and tomato, all for 2.5 euro. It's amazing. Their halloumi, Nil Chicken and Kofte are all great too. There's another location on Oppelner Strasse in Kreuzberg near Schlesisches Tor U-bahn.