Ode to Berlin.

No, really. Some people like New York (cool place) or London (kind of stiff) or Paris, ma chère, but for me the top of the list is Berlin.  You take as look at their public transport map, a nice rectangular shape with  three zones: C is the outlying one which you will want to include if toy wish to visit the Great Frederick's palace at Sans-Souci.  A man who did not like the military but who led troops and never lost a battle, as opposed to his father who lived to play soldier, but never saw any action in any war.  Zone B is very nice, as it includes, in part the Science and Technology museum, tons of stuff for young and old including trains, boats and planes.  Put a good one to two hours aside.  Then you have Tempelhof, the old nazi airport and the now called Tempelhof Freiheit, which the people have tken over as a park.  Tempelhof served as central point during the 24-7   18 months airlift to help Berliners survive the USSR blockade.
Zone A is the most important in a manner as it included pretty much the ''must see'' places in Berlin, especially the nice road from the zoo all the way to Alexander Platz - which was downtown for the DDR people. It also has the radio tower, which you can visit:  top floor for observation point  or one floor below for a nice neal at reasonable cost. In between you also have,mostly on week-ends, used books sellers pretty much in front of Humbolt College and also a Flohmarkt (fleemarket) which follows the meanders ot the Spree and where you have something for everyone.  You also must plan a visit to the Berlin City Museum - also on the same road.  Most pleasant, including aspects of Napoleon's ''visit'' to that city.  He was not welcome of course. Plan a good hour or two.  Worth it.  A visit to the Reichstag is also free-  don't forget your passport.  Tons of pleasant days and activities in a somewhat relaxed environment.  Enjoy.