Either at the airport or before you go buy a travel ticket which covers you on the Subway, trains, buses and Trams. Big saving to me made.
If your going to the wall it's spread out so do your research. Alexandraplatz, postumur Platz both worth a visit plenty round there and parts of the wall.
Checkpoint Charlie another good one.
If you want entertainment Saturday night near Alexandraplatz is a good Bierkeller(not that name obvious style bar) does good food also.
If you get stuck few Berlin based Frankfurt lads who am in contact with be happy to give you some tips.