Welcome aboard, Mary! Your family is going to absolutely love your time aboard the Disney Fantasy. Pair that with the amazing port stops, and you're in for a very magical vacation!
Costa Maya is a great port stop with lots to offer, even if you choose not to take part in a Port Adventure. The port area itself has quite a bit of shopping and dining, and even offers a free salt water pool for cruise guests with a swim-up bar. Many guests not going on a Port Adventure spend their whole day in the port area. There is a beach with chairs and hammocks right there, but it's small and if you're looking to swim, this is not the spot as it is very rocky.
For a nice swimming beach, walk just outside the port area where you can take a taxi to Mahahual, just a short five minute drive away. The taxi will cost around $3 to $5 per person, but prices can vary, so be sure to ask before setting off with your taxi driver. This flavorful fishing village has some little shops and restaurants and a really nice beach that is calm, clean and shallow, so it's perfect for families. There are spots on the beach where you can rent jet skis and take part in other water activities like that. Many of the restaurants along the beach have their own tables, chairs, loungers and massage tables (be sure to check with a few of them to get the best price if you'd like a massage), all on their own little sections of the beach. It's a great laid back, relaxing spot in which to spend your day!
I hope that information helps as you plan your trip, Mary. I'm happy to help with other questions as you continue to plan!