I visited Grounds for Sculpture in Hamilton, New Jersey back in May. The sculpture park is a beautiful place to spend the day. There was a Seward Johnson retrospective going on which included some larger sculptures (like the Marilyn Monroe statue) which usually travel around the country. The colorful flowers and breathtaking landscapes make GFS a wonderful destination for taking photos and even plein air painting. Since the park is open all year long you can experience the sculptures in different ways as the seasons change. Some of the sculptures can be (gently) touched and interaction with the art is encouraged. Throughout the park are three dimensional reproductions of French paintings which would be a great way to introduce children to art history. There are also indoor sculpture galleries with rotating exhibits.
I would love to spend a leisurely day there drawing and taking pictures.