Spent a week there in 2003 and absolutely loved it. One of my favourite places and keep meaning to make a return visit sometime.
I found the atmosphere to be relaxed and friendly. It was almost as if having spent 60 years being told what to do, where and when (first of all by hitler and his goons and then by the russians post war) Berliners are determined to enjoy themselves and leave their troubled past behind.
As for nightlife, I notice a couple have already mentioned Potsdammer Platz for eats and drinks. We also used to go to an area called Hackersack Market (bit unsure about the spelling) which was quite lively. Reminded me a little of market place in Hull, although that was 9 years ago so don't quite know what it's like today.
The nearest train/tube station to our hotel was Belle Vue so I've always associated Berlin with Wakefield in my rather confused mind
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Also, a really nice place to go is Potsdam itself (location of the post war conference which divided up Europe along the lines which lasted until the fall of the Communist bloc in 1990). It's about 40 minutes by train from Berlin but if you're only going for 3 days you probably won't have the time to take it in.
Hope you enjoy your stay, I know I did.
