Usually we visit DH's family in Berlin but I agree with the above suggestions. But one trip we also did the River Cruise for an afternoon and it was fantastic. Here is a link to give you an idea of where it goes etc. I don't know what company we toured with but I know there is a variety of them.
https://www.berlin.de/en/tourism/riv...ses-boattrips/
If you like walking in nature, look into The Pfaueninsel (Peacock Island)
and of course the Olympic Stadium if you are interested in history where in 1936 Hitler opened the Summer Olympics.
Also the Television Tower at Alexander Square ....
If you make it down to Potsam, Sanssouci Palace is a great place to spend an afternoon, lots of steps and tons of history. You can choose tours in English etc. just check it all out online.
Check out maps of where you want to go and how easily most places are accessed on the u-bahn
https://www.berlin.de/en/public-tran...subway.en.html
Do you speak/read German?
http://www.berlin.de/special/shoppin...er-schick.html