4 Michael Hayward - a year ago
If you're ever around on the second Saturday of any month, stop by at the U-Bahn Museum! Very cheap to get in, fantastic amounts on show regarding the history of the Berlin Metro. The main controlroom with the photos and records that surround it as well as the three side-room exhibits led to a good hour and a half of slow looking and reading. Well worth a stop by if you're interested in history.
My only gripe is that they only accept cash, no card. Led to a few return trips to Theodor-Heuss-Platz while I sourced an ATM.