Travel in Germany

As I am in Frankfurt for the next three years, I will use this time to see this country. Already went to München, Schwarzwald, Aachen and visited the towns in the surrounding of Frankfurt. This summer want to go for one week to Hamburg and the Nordsee or Ostsee......obviously cities like Berlin, Dresden, Koln and Dusseldorf will not be forgotten.

Please give me nice tips and nice places to visit ?

Thanks in advance
Steven