There's nothing more revigorating during the dark winter days than spending a couple of hours in a sauna. Germans have mastered the art of sauna and you can find amazing pieces of architecture in the public swimming pools (Stadtbad). I find Stadtbad Neukölln very peculiar in its neoclassic architecture and it's definitely a wallet-friendly option to enjoy some sauna time there. Be prepared to be naked in front of man and woman!
A very good reason to travel West: the C/O, next to the formerly infamous Berliner Zoo tube station, is an awesome museum dedicated to photography and visual media. Previous exhibitions include Larry Clark, Martin Parr, Anton Corbijn, Annie Leibovitz and Stephen Shore.
A good place to drown your Berliner winter-melancholy. The bartenders are top notch and the interior is warm but stylish, with the inevitable retro vibe. Open from 6 till late. There´s also a legendary (and kinda expensive) restaurant next door, which provides the bar with some excellent snacks. Works for dates and for friends, but if you´re planning for a bigger group it might be wise to reserve.
Situated in the patio of the Kunst-Werke, a well established exhibition house for contemporary art, this cafe features mirror-walls and a selection of nice cakes and lunch menu.