The Reichtag offers a free tour around the building which also goes to the top and gives a great view of the city of Berlin. The wait is over two hours long on a nice day. A better way to see the city is walk or rent a bike or by tram.
At Least Do This: If you have to the actuall Reichtag is and awsome thing to see but not worth 2 hour wait. Alternative: have a picnic, play soccer in front of the reichtag on the lawn