I see your kids are 4 and 9 yrs. old, I think the little lagoon in front of the Avalon Reef Club across from north beach would be an excellent adventure and really easy as far as time limits and costs. It's protected from big waves and there are lots of fish to see and it's only 8 feet deep at its' deepest. You can walk into it from north beach for free. On north beach you will find plenty of places to eat and drink, our favorite place is Jaxs which is right on the main drag across from Privleges Resort. They have burgers and other American and Mexican fare for a good price. The kids will love the golf carts, you can't get lost, your on a little island with only a few roads. I would vote for getting a place for 1 night and really enjoy your whole day on the island. Walk restaurant row after your snorkeling and sightseeing, there is something there for everybody! The shops are all open at dinner time and it is so much fun to just walk and shop on Hidalgo Street. You will have the ferry to yourself in the morning/afternoon the next day, a vacation reverse commute