I'm going with some friends for 4 days and I have 2 questions: where could we go for brunch and what else should we do other than stuff our faces?
I just went to Kopps in Mitte for the first time for brunch and it was the best one I've been to yet! Otherwise I quite enjoy the brunch at Pele Mele. For Kopps you need to make a reservations, I think for Pele Mele as well. It's quite easy though and they should speak English if you don't know any German.
Oh La La also has a vegan and gluten-free brunch Sat and Sundays if that's your thang.
As for stuff to do, go hang out at the Tempelhofer Feld. It's a park that used to be an airport and it's awesome and has lots of cure quirky community gardens. Also check out the abandoned amusement park (Google it), it's pretty cool. I general just riding a bike around Berlin is super nice, so it's worth renting one or checking out Bike Surf Berlin to rent an affordable one.