Banteay Srey Butterfly Centre is a pleasant stop on the road to Banteay Srei, with Cambodian butterfly species flying inside a planted tropical enclosure. The centre also works with village families who raise pupae, so the visit has a modest community-development angle as well as the obvious appeal for children and photographers.
It is not a reason to make the long trip by itself, but it makes sense as a break from temple fatigue. Pair it with Banteay Srei, the Landmine Museum, or a countryside loop north of Siem Reap.