I can help!
You do pay for in room gifts when you order them, not onboard like excursions.
You can definitely still drop things off to be signed at guest services. It's a great souvenir. You can do two items per room.
The stingray excursion is fairly short, you can do it and still have lots of time left for something else.
We love snorkeling at Castaway Cay. You can find a decent number of fish out by the sunken objects, especially by the sunken sub and things like that. The underwater Mickey and Minnie statues are great too. It's free if you brought your own snorkeling equipment, otherwise you will have to rent it. Either way you will need to use one of their life jackets, those are free.
They won't run out of snorkeling gear, so you can wait until the day you are there to decide. I'd recommend booking the stingray excursion in the morning and then waiting on the snorkeling equipment (unless you are bringing some). Then once you are there you can decide if you'd rather just relax on the beach, or give it a go!