I loved that city, it has lots of alt places to see because of Heidelberg was the city that the least bombed city in world war 2. The top sightseeing places in Heidelberg is : Heidelberg Castle, Heidelberg University, Altstadt, Student Kranz, Theodor Bridge. You will love walking around the streets in altstadt.
I stayed 1 month in Frankfurt, I love that city but there is lots of homeless people in streets. When you go to the Frankfurt you have to see main river. There is lots of museum you can visit in that city.
I stayed in Germany for 1 month alone to go to the language course. In that time I totaly visited 13 cities