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Besides visiting the butterflies, you can watch an IMAX movie, visit the planetarium or explore the exhibits. The huge new Paleontology Hall is a must see, the Chemistry Hall often has demonstrations and the Energy Hall has movies and interactive exhibits.
7. Splash in the Puddles - If it's not lighting and it's not raining too hard, go outside. You'll probably have Hermann Park and Discovery Green to yourself. Pack some extra clothes and go splash in some puddles.
Another idea is to check the list of current events HERE and find one that is great for a rainy day!