They would probably enjoy the turtle farm (it's a one-time thing, IMO). You could always hang out at 7-mile beach all day, but I would include Stingray City in your plans.
Your little one may not want to get in the water with the rays, but I think it would still be a fun excursion. We used Captain Marvin's and there were only 15-20 people in our group, with two crew members. We also had a family with small children in our group, and one of the crew members brought a stingray to the side of the boat for the little ones to see up close. There was also a platform which ran the width of the back of the boat and the mom and dad took turns sitting with the little ones while the other snorkled. I'd probably do a short excursion though...say 2 hours or so. I know how fidgety a 4-year-old can get.
Here's CM's website....