Hi there wake_tonight and welcome to Flyertalk!

Your question is really quite a difficult one, because it has so many possible answers. From Berlin you can go north, south, east (well, more south than east) or west and all have their attractions. Pretty much wherever you choose, getting there will be easy either by train or plane.

My advice would be to buy yourself a Lonely Planet Europe and use that as your starting point for deciding what particularly takes your fancy for this trip. I find LP is very good at suggesting basic itineraries for different timeframes which, even if you don't follow them, can help you plan your own.

One thing I would say is (apart from visiting friends/family) don't go back yet to places you've been already, as there are plenty more to see.

Once you've picked some destinations, we can give much more specific advice about how best to plan.