Here is a rough plan for how I would do it (with a longer explanation for each time)
9:00-10:00 Stingrays
Definitely keep the 9:00am time slot for the Stingrays!
My daughter did the first time slot last summer and they seemed to interact a lot more at the beginning. By the end of her session, they started to lose interest in being fed, so I would think that later sessions would not be as good as the first session. You will have time to snorkel in the Stingray area so be sure to head out to the edge of the enclosed area, as my daughter said this is where she saw some of the really big stingrays.
10:00 -12:00 Get beach spot-Relax and float- main lagoon snorkelling
If you have time before the Stingrays, you can get your beach spot then, otherwise find a spot now. There are plenty of chairs/lounges/umbrellas so don’t worry that you won’t find a spot - you may just have to walk a bit further.
You get to keep the snorkel equipment all day with the extreme package, so after snorkelling with the stingrays, you may not want to do more snorkelling in the main snorkelling area right away. I would rather do some floating and relaxing next. After a hour or so of this, you will probably be ready to do some more snorkelling and can follow the snorkel path to see the sunken objects and the fish.
12:00 - 1:30 Lunch then relax
By this time, you will definitely be hungry and ready to eat. You can safely leave the equipment/towels etc. on your chairs/lounges while you eat.
1:30-2:30 Bike
After lunch (and a bit of time to digest), I would pick up the bikes to do a ride around
Castaway Cay. Be sure to go to the Observation tower for a good look at the island. Adults in your group can also stop off at Serenity Bay. With the bikes, you can check them out whenever you want, and there are lots of bikes. We had to check them out with a cast member, but nobody checked them in from us so don’t panic if you are not back exactly on time. We always found that an hour was more than enough time for biking and cut it short when the kids were younger.
2:30-until you leave Float/maybe snorkel.
After biking, it was nice to float around to cool off. If you are really energetic, you could do some more snorkelling, but we found that we were pretty much done and just wanted to relax by this time. Then, before you know it, it will be time to head back to the ship.
Have fun on your cruise!