Lori, welcome aboard! I'm glad you convinced your husband to cruise with Disney! Your family is sure to make some very special memories onboard!
If you're sailing to Nassau, you must try one of
Disney's Port Adventures to Atlantis. It's simply unreal! There are a variety of options going there in many different budget categories, from swimming with the dolphins to a fun beach day at the resort. You won't regret seeing Atlantis!
On Castaway Cay, I could spend the whole day just exploring all that the island has to offer and not even doing an excursion. Spending time relaxing in the shade on the beach or playing with my kids at Pelican's Plunge and Spring-a-Leak, and participating in crab races and character dance parties can fill our day with fun. But, if you have some time, try to grab some snorkel gear and head out to the Snorkel Lagoon. Your twins might want to spend some time playing in the sand at Scuttle's Cove with the Oceaneer's Club counselors while you and your husband take your 8-year old out to snorkel. I don't want to ruin the surprise, but keep an eye out for all the magic Disney has hidden under the water among the fish. If you're on the hunt for Hidden Mickey's this is a great place to look!
Snorkel gear can be booked ahead of time, but I've always found they have plenty for those last minute decisions to snorkel on the island. Other than that, I'd certainly take a look at which adventures look right for your family and book them as soon as your window opens, which is 75 days prior to departure, if you're a first time Disney cruiser. Onboard the ship, you can add Port Adventures as well if there is space on a trip you're interested in.
Have a fun time planning your cruise, Lori!