Morris Arboretum of the University of Pennsylvania, Philadelphia

John Morris made his fortune in iron and then, with the help of his sister Lydia, turned to a far more delicate medium: exotic plants. Starting in the 1880s, the duo traveled the world in search of specimens to bring back to their Philadelphia home. That formidable collection blossomed into the Morris Arboretum of the University of Pennsylvania after the siblings donated their estate to the University of Pennsylvania in 1933.

Their carefully manicured English-style gardens, complete with swan pond and azalea meadow, now also belong to the National Register of Historic Places. Visitors today stroll winding footpaths among 13,000 plants, hailing from 27 countries; there's a prominent Japanese influence in several areas, including the Japanese Hill and Water Garden. Stately trees dot the property, including trident maple, Engler beech, and Bender oak—the latter of which researchers believe may predate the Morris estate by more than 100 years. And some notable plants are kept inside: the Victorian-style fernery encases tropical fronds and trickling waterfalls in a balmy paradise of glass.

Children especially flock to the Garden Railway, where G-scale model trains chug through a miniature garden, passing buildings made entirely of natural materials by skilled chipmunk carpenters. The Garden Railway runs from Memorial Day through Labor Day, and on weekends through mid-October. They and their minders also tend to enjoy scrambling across wooden bridges suspended high above the forest floor to get a bird's-eye view of the arboretum at the Tree Adventure.