While the St. Louis Zoo gets my vote for Best Attraction, the Science Center wins, for me, as best Family Outing. This is solely because while, like the Zoo, it's open year round...it is also indoors. That makes for all weather family fun!! The Omnimax Theatre is wonderful, and I couldn't imagine having more fun learning!
Pros: It's free, Ominimax is great, Informative & Fun
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