If you're looking to mostly stay in Germany, check out the German rail passes (for tourists, long distance express trains included) and if you're not traveling until April when the Deutschlandticket (finally) comes into effect, then this is a more affordable option for a whole month of rail travel in Germany for only 49 euros, but intercity trains are excluded.

Like the previous reply said, no OW partners serve CGN, but if you want to fly direct into secondary European cities, check to see if you can conjure up an itinerary with BA/AY/IB (unfortunately with transits all out of the way for travel to Central Europe, unlike ST and *A options.)