To be quite frank I have found that when the kids range from the age of 5 months to about 6 years old the best, most fun activity in the world is sitting at the shore line with a plastic cup. They use it to catch fish, pour water over themselves and others, it can be used as a shovel, as well as old party favourites- prancing around acting like it is a hat or a bullhorn.
I believe the butterfly farm would be fun for him, but the zoo is another matter. I guess you know how patient and/or adventurous he is. Many times when we went to the zoo it was hot and the animals were all sleeping and camouflaged which meant that the little ones just couldn't see them. You can't tell a toddler to look to the left of the 3 ft long elephant ear. I was always nervous about them sticking their hands in the cages too.
You might be best off considering this a relaxing vacation, hanging out by the pool and the sea, wandering the beach making friends with other families with toddlers. Or consider having a baby sitter for the times you want to go diving or visiting the Maya Ruins etc.