I would definitely recommend a visit to the Reichstag building when visiting Berlin. Even if you're not too interested in the historical or political aspects, the building and the views are amazing. And, it's free, so what's not to like?
You have to register and book your visit, but this is easy to do on their website. They advise you to turn up 15 mins in advance, in order to allow time to get through security and you need to bring your passport.
A lift takes you up to the roof terrace and dome, where you are given an audio guide, which is available in several different languages. Toilets are also here.
The information boards provide a background of the German political system and Reichstag building through the years and once you start to make your way up the spiral to the top, the audio guide kicks in and explains the views as you walk. You can get some really good photos across the Berlin skyline from here, as well as on the roof terrace.
I like the fact there is one way up and one way down, so you're not meeting people.
There is a little gift shop counter on the way out where you can buy souvenirs.
We spent about 1.5 hours here and would highly recommend a visit. Still very surprised it's free.