Hey there, here are some cool things to do in berlin that are not too touristy :)
Activities
~ Herzberg Golf (really pretty minigolf place in Neukölln, if you go in the evening the lights are amazing)
~ Prachtwerk Neukölln (small venue where local artists perform)
~ Berlinische Galerie (museum for modern art in a nice neighborhood)
~ Tempelhofer Feld (used to be an airport, now its just a giant field where you can skate, picknick or just walk around - i think you can rent skates or bikes there, not sure tho)
~ Humboldt Forum (art museum in an old castle, very impressive structure and they have changing exhibitions - this one is not very unknown but worth a visit)
Food
~ La Maison (french bakery in Kreuzberg, a little hipstery but they have amazing pastries!)
~ La Bolognina (handmade pasta, very delicious :))
~ La femme (turkish cuisine, they have multiple restaurants in berlin - the food is heavenly, especially the pide)
~ Sahara Imbiss (for a quick snack, sahara has amazing falafel/halloumi etc. for 4-5€)
Bars
~ Villa Neukölln (cocktails and sometimes livemusic, can get pretty crowded tho so come early)
~ Zum starken August (amazing interior, they have good cocktails and sometimes burlesque performances)
I think thats it for now, hope you have a nice time in berlin :)