I live in Alexanderplatz and I work in food. The area is mostly tourist traps (bad ones at that, we are in Germany, not in Italy). If you want something very casual, Bahn Mi Stable is the closest decent thing from Alexanderplatz. If you want a proper sit-down restaurant try Trio (German), Soopoolim (Korean), or Pizzeria Standard on Torstr.
Ignore completely the reviews on Google Maps or tripadvisor, they are totally unreliable.
If you want touristic stuff, the city center is quite boring but you have a lot: museum island, branderburgertor, Gendarmenmarkt, Checkpoint charlie and all the surrounding areas. If you want something more interesting, the Soviet Memorial in Treptowerpark, Victoria Park, the various memorials in Hellensdorf. Also avoid at all costs the DDR museum, lamest waste of money you can think of. Other museums in the center are ok.
It's very poorly written anti-DDR propaganda. You must obviously expect to be fed propaganda in a museum like that, that's the purpose, but it's very... passive-aggressive. The tone is not very rigorous and after a few years it got pathetic because the things they use to make fun of the DDR, today would be considered middle-class privileges.