I'd suggest a visit to the DDR museum just next to the Cathedral church very close to the museums! This is tiny and interactive but leaves a lasting impression.

Also nibble a currywurst from the stalls opposite, near the Opera House.

I was NOT thrilled with the Checkpoint Charlie Museum ... far too pricey and a very obviously commercial enterprise, nor with the 'fake' soldiers outside and at Brandenburg Gate which I felt demeaned those who had lived through this dreadful period of history.

I recommend the Jewish Museum, even if you are not Jewish, and the Deutsches Historisches Museum which has a very honest and fair take on the Nazi era.

The Hauptbahnhof - not usually considered as a tourist site - is an amazing construction with plenty of shops and refreshment facilities.

The Hop On Hop Off bus with its 24 hours' validity of tickets means that if you take the last trip of the day, you can do it all again for free the next morning.

It might be worth looking into getting a Public Transport Pass ... ticket office at Tegel airport.

We also enjoyed the Charlottenburg Palace for a bit of fresh air and 'frothier' history.

A wonderful city - which will delight and exhaust you!