We had a very knowledgeable guide take us through both the combined reptile and butterfly day tours. The combined trip was probably worth the extra cost, but the enclosed butterfly area was utterly awesome. They show you how they cultivate the eggs & pupae in their lab and then go through the butterfly enclosure - quite large and teeming with beautiful butterflies all over the place. You can hang out there afterwards, or even come back later, to just watch all the colors flitter around you.
A bit inconspicuous from the road - look for the hotel and souvenir store on the right as you had south toward Manuel Antonio. There's parking on the left and a small path going into their entrance.
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