While this may not be that "hidden", I have found many people in Berlin don't know about two great venues in Kreuzberg: the Burgermeister and Madame Claude. Visiting the two in a single night makes for a quality yet cheap plan, provided you like burgers and alternative / experimental music.
Madame Claude is the greatest bar I've been to so far, not only because of its friendly, surrealistic atmosphere but also because of its cultural offering, as in: experimental music on Monday (the so called "Experimontags"), a music quiz on Wednesday, pretty interesting live music on Friday and Saturday and a singer/songwriter jam session on Sunday.
Tip: The last.fm page is the most updated one if you want to see upcoming events.
Tip 2: Buy a beer and wait around the "live music room" if you want to get a decent seat.
So this is my typical Monday night "happyness recipe": Go to U1 Schlesisches Tor, have a burger at Burgermeister for 3.5 EUR, head to Madame Claude, pay a couple of EUR for the entrance, get a beer and watch the artists of the "Experimontag". All together, 8 EUR.
And if you don't care about budget, but you like good food and alternative music, you'll be happy too!