Berlin guide tips? (Luxembourg)

Hi,

I'm going to Berlin next week for 2.5 days and I've never been.
So if anybody in here has been there and have some good insider knowledge/tips what to see or do I would be very grateful.

I'm looking for good restaurants, cafés, attractions and areas that maybe only people living there knows about or where it will not be too packed with tourists.

Cheers,
Benjamin