I (Emily) traveled to Berlin at the end of my study abroad in Germany, back in 2015. Like any large city, there were several parts under construction/renovation, so if you see pictures with construction, know that it's probably done now.
Our first day there, we took a bike tour around the city--hitting up much of the architectural and historical highlights of the city center. A few of the stops included checkpoint Charlie, Museu Island, Brandenburg Gate, the Berlin Wall, Reichstag Building, Alexanderplatz, Alte Nationalgalerie, and the New Synagogue Berlin.
If you're in the city for a short time, I'd recommend doing a bike tour or renting a bike for the day. It's a great way to see a good portion of the city quickly.
One of the days we visited an art galler called Hamburger Bahnhof, which houses a few collections of contermporary art. Originally, the building was used as a railway station (which is why it’s given the name bahnhof), connecting Hamburg to Berlin. Today, it is the last train station standing from the 1940s.
None of us really enjoyed the art inside, and I personally wouldn't recommend it to any one, but if you're into odd, contemporary art, then this might be a stop for you.
They take security very seriously here--you will have to go through what I call "airport security": metal detector, bag scanning, the works.
Tourists are permitted to go to the top of the building, where there is a glass dome with a fantastic view of the city. Keep in mind that glass rooms act like a green house, so if you go in the summertime the inside of the dome will be very warm.
For the architect lover: Konig Galerie/St. Agnes
This is a former church building converted into an art gallery. The structure has a Brutalism style which was given to it after it’s restoration some years back.
The museum displays Jewish history and culture and shows the migration of Jews throughout Germany and parts of Europe. From the outside the structure appears to be a silver zigzag--walls completely made from metal and irregularly shaped windows riddle the walls. Inside the building, the walls and windows are all slanted, and the visitors feel almost like they are within a maze.
One of the days we visited “Myth Germania,” which was Hitler’s plan for a new Berlin (which was planned to be renamed Germania). Their we viewed Edgar Guzmanruiz’s model which showed the overlapping of the planned city, and what we see today.
Sachsenhausen Concentration Camp
Built in the summer of 1936, this camp served as a model for the other camps, giving it a special role in the National Socialist concentration camp system. The prisoners it housed at first were mostly political opponents of the Nazi regime, but as the war raged on, those who were seen as racially or biologically inferior were also included. Overall, over 200,000 people were held in Sachsenhausen at one point or another, and tens of thousands died or were executed within its walls.