I've just got back from the Qt Contributor's Summit, and I had a really good time.
I arrived on Wednesday evening and we had arranged to meet in a bar called 'Brauhaus Lemke' in Hackescher Markt which is quite near Alexanderplatz. It did look easier to find on the map than it actually was, but Hackescher Markt is a great place. There is a big square with loads of bars that have seats outside. The Lemke is slightly off the main square.