We enjoyed visiting Museum Insel as it gave us a choice of what to see. A must, as a previous post has said, is the Pergamon Museum. We also found the film museum fascinating and you can rent audioguides in English to help you enjoy your visit. Although there is a lot of focus on 20th century history, it is not the be all and end all of the city and you will find something to suit most tastes, including a relaxing trip on the river if the weather is nice.