While cruises can be a lot cheaper than other types of vacations, they still come with a lot of fees. When you get to port, odds are you're going to want to spend some time just relaxing. You can take a stroll through the beach town or simply stay at a free local beach all day. However, this one requires a bit of research as most cruises have their own areas where they charge an entrance fee.
You can check on social media and travel sites or just ask around when you get there to find the best free beach. It might require that you take a taxi, depending on how far it is, though.