My DH and I thought the same thing when we were looking to get away for a few days, a few years ago. We had, at that time, been on one 4-night (first cruise back when sons were small to see if we all liked it), and several 7-night cruises (turns out everyone loved it) with the kids for our family vacation. With the kids, we were active all day, going from activity to activity, movies, pool, games, etc Wonderful trips, but not a lot of rest time for us. When we saw an opportunity to get away for our first "alone" trip in years, we decided on a 3-night
Disney cruise. We went thinking it would be ridiculously short, and that we would feel like it was over in a blink. Turned out, we stayed on the boat during Nassau day, went to Serenity Bay on CC, relaxed, actually talked to each other, saw movies, ordered room service, even skipped the shows that we had previously seen. Dressed up for dinner each night, and acted like adults. We were shocked at how leisurely it was, and how long it felt, considering it was only 3 days. If you are travelling with your kids, then, I would definitely suggest to try to make it a 4-night, though. Its a big difference when you are wanting to do many activities, and with the kiddos, you dont want to skip anything.