The Philadelphia Zoo is a pretty good zoo by most standards. Compared with the Bronx zoo and the DC zoo, it is slightly smaller, but it does have some unique aspects. It is very easy to get around with kids, the specialty animal houses have some species of monkeys and reptiles I've never seen before. There is also a hot air balloon ride for an extra fee. The downside is the cost: $17 for adults and parking: $10 to be a 5-10 minute walk away.
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