Cave tubing does require you to leave the island but its organized well, leaving the island and returning to the island, as a day trip.
Good thing about cave tubing is Grandma and a 5 year old can both do it and have fun.
My guesstimate of nice dinner at one of the better restaurants for two people and drinking..150.usd.

White Sands Dive Shop
https://whitesandsdiveshop.com/