In the age of the smartphone, getting kids excited about nature is increasingly difficult, says Alan Metcalfe, principal of Philadelphia-based Metcalfe Architecture & Design (MAD).

So, when the Morris Arboretum, which is owned by the University of Pennsylvania, approached MAD about designing a child-friendly walkway on its grounds, 50 feet up in a stand of oak, beech, yellowwood, and maple trees (among others), the firm jumped on the opportunity.