The Jardin des Plantes isn't just a pretty place to spend an afternoon. From its “humble” beginnings as King Louis XIII's herb garden, it has grown to well over 7,000 plants. In addition to being home to four museums and a zoo, it's also a working laboratory for a highly respected botanical school.
The gardens feature native French as well as worldwide species of decorative plants. Of particular note is the Rose Garden, at 22 years old, it's the “newest” garden in the collection; its heavenly view is bested by the heavenly scent of thousands of roses.
The Jardin des Plantes is in the fifth arrondissement, with easy access via the M5, M7 and M10 lines.