I've been to Berlin more times than I can count on my fingers and toes! Most of my favourite restaurants are in Kreuzberg which is where I used to live. My favourite street in Berlin is Bergmannstrasse (closest subway stops are Gneisenaustrasse or Mehringdamm) and you will definitely find something very delicious there.
My favourite restaurants on Bergmannstrasse are Pagode (cheap Thai food!), Good Morning Vietnam (this one is a bit hidden off the main street, but once you find it they have a beautiful terrace on the roof. It's perfect for a summer night.) and Parlamento Degli Angeli for Italian.
Curry at Good Morning Vietnam
Also just around the corner is the famous Mustafa's which is a little Doener stand that always has a massive line-up. You have to try Doener when you are in Berlin, it is soooo good! If you don't want to wait an hour at Mustafa's, I would recommend going to Yorck (near Mustafa's) or Doener Box (right outside the Heinrich Heine subway stop). Both these shops look a bit "rough" but their Doener is so yummy and authentic!
If you're in the mood for a fancier German dinner, go to Borchardt in Mitte for their Schnitzel.
If you have time to explore Berlin by day, you have to go to Cafe am Neuen See which is near the Berlin Zoo and is just a beautiful place to relax by the water! Museum Island is also a must -- I would recommend going to the Alte Nationalgalerie or the Pergamon. If you want to go shopping, head over to KaDeWe (the Harrods of Berlin) or any of the shops on KuDamm, which is in an area called Charlottenburg. Also Dean and David's is nearby for yummy salads. This is a fancier area but really beautiful! If you want to grab a bite around there, I would go to Savignyplatz for authentic German at Dicke Wirtin.
Salad from Dean and Davids
My favourite bar for after work is Kitty Cheng, and DuDu is nearby for sushi as is Hartweizen for amazing spaghetti carbonara! Also, Berliners love their beach bars -- Deck 5 is my favourite, it's on a roof but you feel like you have been transported to a beach!
Sushi at DuDu
Spend a summer day at the swimming pools at Badeschiff or Haubentaucher, or walk along the East Side Gallery. Try to get into Bargain (if you're into that!), and don't forget to visit the Bundestag, Checkpoint Charlie, The Holocaust Memorial and The Brandenburg Gate.