We came across this jardin serendipitously. That happens a lot in Paris--you're on your way to a…read moremuseum or restaurant, turn a corner, and *poof* you are somewhere cool. It is absolutely worth planning to stroll through these jardins, though, and close as they are to the Louvre and other landmarks, there really is no reason to not plan.
Around the perimeter of the jardin, there are some lovely little shops to peek into. Art, antiques--many things to admire! At the far end, the one closest to the river, are the famous striped columns that you see in so many photos. (Yes, they are totally Instagrammable!)
If you would prefer something calmer, stay on the far end, which is quiet and green. It's really a nice little pause.