Berlin is awesome :) So many parks and monuments to explore there! My advice: if you go out try to go into one of the clubs at the Alexanderplatz which has a terrace at top. Then party till the morning and wait for the sunrise. Did this with a friend on our interrail trip - it was amazing! Maybe I can find the pictures somewhere on my old laptop.
If you happen to travel to Austria as well --> There you could get a nice refresh after an exhausting city trip, check this mountain lake f.e.: https://steemit.com/nature/@theaustrianguy/a-beautiful-sunday-hike-to-the-laudachsee

Greets :)