Okay, Berlin literally has a museum for everything, so stop here first and look up things according to your interest:
https://www.museumsportal-berlin.de/en/museums/ Favs of mine include the Bauhaus Archives (currently closed for remodeling but have a temp location in Charlottenburg neighborhood and the Berlinische Galerie which is a modern rt museum which also has a lot of Dada and Avant Garde in it. If you prefer to kick it old school and see older things Museum Island is where you want to go.
Two things you should eat there: Doner kebab and currywurst. Both are delicious and are everywhere. Also try the Burger King and Dunkin Donuts as there are items on the mmenu that are not available in the States; the banana iced coffee at DD is not to be missed.
Neighborhoods to walk through: Mitte, Kreuzburg, and Prenzlauer Berg. They are all in East Berlin. Kreuzberg is a nice mix of everyone and you can get good doner there while Mitte and Prenzlauer Berg are just hip areas to walk in and shop. Alexanderplatz is in the middle of Mitte and is like the town square where all the trains, buses, and subways go. You can shop there, and there's a shopping mall in the subway station so you can get food.
Books: Charlottenburg is in West Berlin and have good bookstores.
Club stuff: Too many to mention, and something for everyone If you're into kinky and want to visit a sex club they have that as well. Be advised that a lot of clubs don't close until 6-7 a.m.
Areas of interest: Checkpoint Charlie area is fun but watch your belongings over there as it's very touristy. The Jewish memorial is somber and beautiful. Brandenburg Tor is great for Instasnaps. The Topagraphy of Terror is educational (it's on the former grounds of the main SS building in WW2)and creepy at the same time. The sections of the Wall that are left is now an art gallery and is awesome.
More stuff: Basically go to this page and read everything:
https://wikitravel.org/en/Berlin I hope you enjoy your trip!