On our Eastern Europe trip last year, the host wore jeans and the hostess, a house dress. It was quite informal. I would dress for the weather.
As for the weather, go on www.weather.com a few days before departing, do a 10-day forecast for the cities visited, and pack accordingly. Even then, there is no certainty that the forecast will be correct, so be flexible. We just returned from the Great Rivers of Europe cruise and the weather was much warmer than forecast from start to finish.
Finally, most people do not dress up for the Captain's dinner. If your return to the ship is late, you may not have time. If you do have time to clean up after the day's activity, don't feel that you must wear a coat, certainly not a tie, and for women, perhaps slacks and a nice top.