I am a sucker for scavenger hunts. I think they are great ways to sneek education into a fun game. Visit their website before going and print off their scavenger hunt. We're not from Texas, so I also bought a child's guide to the state (one of those 16pg state facts books) to help him with some of the answers. We loved it!
The butterfly house is worth the extra money (though we didn't go this last time, we've been before in the summer and it's amazing).
We loved the Scent garden, though it's been re-done and its not longer just a scent only garden (its been updated and I think made prettier while still promoting the idea of a scent area with strong smelling herbs, plants, flowers). The Rose garden was my personal favorite.
If you're a local, they also host a lot of workshops and camps for school age children and adults that look interesting.
Seven-acre garden area, botanical collections, and a special Garden for the Blind, featuring plants noted
for their textures or scents.