Corinthian Gardens is a vibrant Fairmount community garden in Philadelphia next to Eastern State Penitentiary. It is free and open to the public from March-October. The garden spans a third of a block and is maintained by a crew of friendly volunteers. The view of the Philly skyline here alone is worth the trip.
The Friends of Eastern State Penitentiary Park have a Facebook page listing free upcoming events such as yoga and park cleanups. They even welcome visitors to take a little taste of their herbs, veggies and fruits.
The garden has a shady natural sand box, play space and picnic tables. There are colorful activity stations tucked around the gardens to encourage active exploration of the many garden areas.
Dogs are not allowed, and the garden is carry in and out. Check out the Little Free Library box as you leave. 
