Go to the bar, have a drink, look at the sea, have a drink...
Seriously, there was very little in the way of entertainment, although a few ferries may have had a cinema. Otherwise it was mostly food, which I think tended to be from a cafeteria (very modern...then). You bought your duty frees or one of the range of gifts: both offerings were limited. You would also have to collect your landing card. As time went on a few things like one-armed bandits appeared. Many people would just bring a book and a blanket, and read their way across.
As a personal opinion, I would say that the period, particular the 1960s saw considerable changes as the ferries changed from just being a means of crossing the Channel to something that was more of an attraction in itself. I can only remember the late 1960s.