A bit late but maybe for the archives...
It depends what sort of thing you are looking for. For day rides then its quite nice heading out to Müggelsee and the other lakes out there is quite nice. Also quite nice is the route to Zehdenick or out towards Potsdam. If you want multi-day rides then best heading out to Brandenburg - either by bike or skip the city by taking an S-Bahn or RB to begin with. Have a look at
OpenCycleMap or the
Brandenburg Tourist Board for suggestions.