I have traveled to Frankfurt for work at all seasons so can address the weather question at least. If you are referring to high temps, it would be extremely unusual for it to be below freezing during the day in March. My memory is that temps are typically in the neighborhood of 45-50 at that time. By April, the weather is always very spring-like. I think that lows might dip below freezing in the middle of the night, but I'd say that the March weather is similar to a northeastern US city like PHL.
I'm not much help on tourist activities unless you need advice on meals or beer. You would probably be better off to ask this question on a different forum.