The love parade happens at that time.
Checkpoint Charlie musuem is a must and the Russian war memorial is pretty cool. Go do some nude sunbathing in Victoria Park. And the old Gestapo headquarters is worth a look. River tours, the 100 bus, rent a bike. Go walk through the Brandenburg Gate down Unter Den Linden to the TV tower.
Oh, and watch out for people having a good time. There’s always a danger of running into people dancing around fires in the parks, raves in ATM lobbies at banks, unlicensed ABBA discos in people’s attics, Sunday jazz bands in great little pubs, massive nightclubs, music festivals, random parties, cellar bars. Berlin is madness. I lived there for nearly three years and don’t remember any of it.