My sister has lived in Berlin for nearly 30 years, I go there 5 or 6 times a year.
The best restaurant, which you must book, is
https://www.katerschmaus.de/
Also, try an go to Burger Meister in Kruezberg at Skalitzer Strasse.
Head to Hackersmarkt, go down the lanes behind the main buildings and you'll find the cafe culture, with great places to eat fantastic cakes.
Then, walk a little down the road to Orienburger Strasse and visit the New Synagouge. It's incredible and tells both the poingant story of the sorry past of Berlin as well as the modern take on protecting the future.
https://en.wikipedia.org/wiki/New_Synagogue_(Berlin)
Be sure to look around the pavement for the brass plaques that tell the story of the past.
Lastly, the city is built just down river from some of the most amazing lakes you have ever seen. Head to Eichwalde from the airport, book a boat trip, and be amazed. Most people that visit don't realise that just a half hour out of the city are lakes that make Windermere look average.