Jealous! Berlin is without question my favorite city and I can't wait until the next time I'm there.
The first thing I always do when I arrive in Berlin is buy a Berliner Kindl from a Spätkauf and dig into some Currywurst from a good Imbiss. My favorite Currywurst Imbiss is unfortunately out of business. There's a good Imbiss right beside the Currywurst Museum (yes, a Currywurst Museum) which has spicy curry ketchup that's actually spicy. Konnopke's in Kreuzberg would be a good idea to visit too because it's supposed to be one of the oldest in Berlin.
To be honest, I can't really give recommendations to German food places in Berlin because 1) I never ate traditional German food while living in Berlin 2) Berlin isn't the city for that. It's joked quite often that Berlin, despite being the capital of Germany, isn't really German. It's a very international city and you'll probably find yourself on the U-Bahn with 6 different languages being spoken in the train car without one of them being German. There's quite a few Sudanese Restaurants in Kreuzberg that I can't recommend enough, and there's a great Ethiopian place right by where I used to live in Neukölln.
As for other things to do, if you're there on a Saturday, visit Mauerpark. There's a big flea market on Saturdays with great food and they have a big open air karoke. If you like theater, then take advantage of it in Berlin. It's very affordable and some of the best in the world. Berliner Ensemble is my favorite. It was founded by Bertolt Brecht and lives up to the legacy. I've probably seen six or seven pieces there. Unfortunately, it's out of season when you'll be there, but maybe you'll get lucky and they'll play an off-season show. I would also recommend you ride on the U1. Most of it is above ground and some of the views are gorgeous. Something else cheap and fun would be to buy yourself a couple of Berliner Kindls and stroll down East Side Gallery, where the wall is still standing and is covered in art. On that point, another highlight of Berlin is the street art and graffiti. Some people hate it and see it as destroying the city, but I love it, and street art/graffiti actually began in Europe in Kreuzberg after being brought there by Americans in the sixties from Philly, so Berlin is the place to appreciate it. I should also note that street art and graffiti in Germany doesn't have the gang and violence connotations that it does in the US. Look up Berlin Kidz on Youtube if you're interested.