Wandering around gorgeous plants and trees is an easy way to boost anyone's mood. Explore the displays at Jardin de lEtat and be sure to stop and smell the flowers.
You'll have no end of sightseeing choices in Saint-Denis. Jardin de l'Etat is a good place to kick off here, but there are lots of other worthwhile places to enjoy. La Barachois, Petit Marche, and Aquanor are not far from the showcase of plants and will fill in your day nicely.