I agree with some of the other comments and would skip the Butterfly farm (not much to see) and I would also save a trip into Oranjestad for another day (by public bus - cheaper/more convenient). Indeed, I would save the car for activties/sites that you will really need it for. I should also point out that if you are going to the Natural Bridge, you may as well stop at the Bushiribara ruins (it is very close to the Bridge).
You should also keep in mind that many companies offer tours of the island that offer similar stops to those mentioned in your "game plan". These tours are of the full day variety (i.e. 9:00 AM to 4:00 PM) without a stop at the Butterfly farm/in Oranjestad. Hence, you may want to be careful that you do not pack too much into your day.
P.S. make sure that your rental vehicle is a 4X4 if you end up going to the Natural Pool (a compact rental car will not get you there).