One of the best urban oasis in Paris is Parc Monceau in the 8th arrondissement.
It's so beautifully designed. It's unique that upon exploring the gardens, you will find a series of "follies"- structures that look like ancient ruins from around the world scattered throughout the garden.
Unusual for a garden in Paris, it's laid out in the more haphazard English style, with winding paths and enormous old trees, and a much less manicured look. On a sunny day, the lawns are packed with sunbathers and teenagers drinking Kronenberg, and about a million children.
