Rouen's Jardin des Plantes is the city's main botanical garden, a calmer southern counterpoint to the tight medieval streets around the cathedral. The grounds mix formal beds, mature trees, lawns, and glasshouses, with enough variety to make it more than a neighbourhood park.
It is most useful as a slow stop between heavier museum and church visits, or as a picnic break if you are staying south of the centre. Spring and early summer bring the best colour, but the greenhouses and tree cover make it a reasonable breathing space through much of the year.