I'm not much of a cruise guy myself, but my aunt & uncle did the Danube river cruise from Budapest to Vienna last November and love love loved it.
We were coincidentally in Budapest like a week or two after their boat sailed, and I really liked that town. Might be the only major city in continental Europe where absolutely no one faults you for not speaking the language (Hungarian might as well be from another planet). We just rented an AirBnB apartment and walked around for a few days.
