I played the horn in high school and college bands and orchestras. It's a wonderful instrument to hear and to play.
Our Elbe Christmas cruise was in 2014 on Grand Circle from Dresden to Berlin, and we spent extra time on our own in Berlin, Leipzig, and Lubeck. Depending upon the timing and your interests, you might be able to find special music in some of the churches. We heard a brass concert in Berlin and Bach's Christmas Oratorio in the Meissen Cathedral and also in St. Thomas Church in Leipzig. Prague will have a lot of music with signs outside churches and other venues.
Foodwise, Dresden is known for stollen, and Berlin is known for currywurst. We tried currywurst - I thought it was awful, but my husband liked it. There is a curry-flavored sauce on the wurst.
The cities and markets on the Elbe cruise are mostly less crowded. Dresden's market was jammed, but we were there on opening day in November with its many special events. The Berlin market at Gendarmenmarkt was especially pretty with white tents. Other markets in Berlin seem to specialize in specific types of goods. We stopped at another very small market in a small town, but I don't remember what town it was.
The weather was a bit colder on the Elbe than in the other parts of Germany that we'd visited other years, but it's variable. My husband bought a much needed wool watch cap at one of the stalls on the Elbe. We went without coats in Munich once just before Christmas, and we needed scarves and several layers at other times. We like silk long underwear for extra warmth. We never had heavy rain, and we never saw snow, but we were ready with rain gear with hoods. The coldest we ever were was in Budpest at the end of December - windy and bitter at the Fisherman's Bastion.