One of the fun activities that goes on during the Disney Cruise is the Castaway Cay 5K. I have done this 2 times and really enjoyed myself each time. The run is free but you must register for it at the Port Adventures desk. If you are looking for a fun way to see the island before your relaxing day on the beach I would highly recommend this Fun Run. You can walk or run this and the course is pretty simple. It takes you through the bike path including the loop by the water tower, up the road toward Serenty Bay and back.
This will be an earlier morning. Order room service or get to Cabanas early. Make sure you meet at the assigned place noted on your registered ticket 10 minutes before the report time. They load off the ship prior to all the other guests.
Pack an extra bag with your swimming suits, flipflops and sun screen so you don't have to walk back to the boat. There are bathrooms to change into swim clothes after the run.
Also, if both adults want to run, Scuttles Cove (Kids Club on Castaway Cay for kids 3-12) is open for kids to join right before the race.
Once you complete the race you're given a medal. Don't forget to stop by thestore right by the finish line. It is the only store with all theCastaway 5K merchandise.