I wouldn't give peppermint. In humans it can relax the lower esophageal sphincter and cause heartburn. Camomile is supposed to promote relaxation. Birds, in general, are more sensitive to these sorts of things.
If you want to give them a special winter treat I would give them slightly warm mealworm water. Or veggies like chopped tomatoes and lettuce in slightly warm water. Or their regular food in slightly warm water. Perhaps slightly warmer than room temperature.
Personally I would be very careful NOT to give hot or too warm water. One time I saw a caregiver let their parrot drink soup and the parrot got a crop burn that was about 3 inches long. It burned right from the inside to the outside of his crop. He got stitched up, under anesthesia, and lived, but barely. I don't know if that can happen to adult ducks or not - but seeing it happen to a young parrot is enough to make me say something.