Welcome aboard, Heather! Your family will have such an amazing trip this fall on the Disney Fantasy for the Eastern Caribbean sailing!
I checked with an agent at Disney Cruise Line about the timing for these Port Adventures in St. Thomas/St. John. It looks like guests can usually begin heading ashore around 7:45 a.m. However, the Sky Ride doesn't open until 9:00 and by then, you'll be heading off on your
Magen's Bay Beach Break. The agent told me that normally, guests leave the beach, boarding their transportation back to the pier around 1:30 or 2:00. That would put a guest at the port around 2:30. Since all-aboard time for the ship is normally around 3:45 that day, it would be awfully tight to head off to the Sky Ride after your first Port Adventure, Heather.
The
Sky Ride is different from other Port Adventures in that guests must find their own transportation to and from the ride, so most likely you'd need to get a taxi. Add that time with the actual time riding the Sky Ride and enjoying the scenery from up top, and you might not make it back to the ship in time.
If it were my family, I'd probably just opt to experience Magen's Bay and have a relaxing day, knowing that Disney's Port Adventure will safely return us to the port with plenty of time to board the ship. There would certainly be a bit of time for a little shopping at the port area once you arrive back from your adventure, Heather. I hope that helps as you plan your adventures with Disney Cruise Line! Let me know if you have other questions!