I think indeed that staying at the beach in Samui is the best you can do. As you already know, we've been there a few weeks ago.
There is a Crocodile Farm including a Snake show and other small animals, but it is mostly disapointing (small cages and dirty croc pits) and also very expensive. You can skip that with no regret.
At the Na Muang Waterfalls, there is an Elephant Ride with a small zoo adjected. And it is the same: Overpriced and bad exhibits (and no rarities as well).
Only the butterfly park in the South of the Island might be interesting. But according to some reviews (tripadvisor), you have to be there in the morning or at times where it isn't too hot (whatever that means in the tropics

).
Keep safe and enjoy the rest of your stay.