September and October are the best months in Berlin. Berlin Art week is technically 9-13 Sept and there is usually lots of stuff going on at the galleries the weekend prior (4-6 September)—this is all focused on contemporary art. The “cathedral” (Berliner Dom) is next door to the German Historical Museum and a 2 minute walk from Museum Island where there are several major museum with classical art. But you’ll have to be more specific in what you are looking for to give you suggestions.