Ages and health will determine what some are capable of doing. Wife and I are 53 and we don't opt for the adventurous excurions - we like to relax. We went to see the Dzibilchaltun Ruins at Progreso because it gave us a really good taste of the Mayan culture without taking up the entire day. The ruins are not huge, but still very interesting, and the entire excursion lasted about 4.5 hours. That left plenty of leisure time to stroll around town or do shopping. We love the beach day on our own at Cozumel. We really like taking a taxi to the beach and relaxing, swimming some, eating on the beach and people watching. We have gone to Paradise Beach twice(great food and drink) and thoroughly enjoyed both times. Chankanaab Park is also highly rated. There are so many excursions to choose from, it might be hard to pick one...so you will have to book another cruise later to do something different!
Have a blast!