I've never been so pleased as the moment when, after several hours trying to hold a disto still while surveying chilly low-airspace crawls in Ireland, my companion produced a hitherto unmentioned flask of hot wet (although that meant we then had to do another several hours afterwards!).
I took a flask of mulled wine for an underground walk with my housemate shortly before Christmas, and that was very lovely, imbibing surrounded by pretty white stal. But I don't generally bring anything to drink at all unless it's going to be a particularly long trip.