Are you referring to a shore excursion booked through the ship, or to making any kind of plans while in port? I almost always get off the ship when it's in port - with the exception of my Disney Dream cruise in February when we were in Nassau (my 4th time there). The few times I made no plans, I ended up wandering aimlessly around the port area - and I don't recommend that!
Learn about the ports you'll visit before you leave home. If nothing appeals to you and you choose to stay on the ship - great! However, don't be like some clueless people who get off the ship, look around the port and don't like what they see, then conclude that there was nothing to do there - simply because they didn't bother to do their homework.