September 9-11 Les Traversées du Marais is a three-day music festival with a "Carnival!" theme taking place throughout the historic courtyards of the Marais district, see the map here.
September 16-18 You'll find food specialities from Southwestern France at the 17th annual open-air food market on the Seine, Les Marchés Flottants du Sud-Ouest, for three days along the Quai Montebello (on the Left Bank, facing Notre Dame Cathedral, M° St Michel). Free entry. Open 10am-10pm (Sunday until 7pm).