Growing up in the area of Munich and living now for over 10 years in Frankfurt, I would second the recommendation to shorten the stay in Frankfurt to 1 day and spend the rest in Munich (and Nuremburg).
In the area of Munich, I can recommend not only the obvious (e.g.
Herrenchiemsee Palace), but also locations which are less known, e.g.
Burghausen which has the longest castle in Europe. Both locations can be visited on a day trip (one each) by public transportation from Munich. PM me if you need further assistance.