Keep an eye out at Marche des Quais — you're sure to find some treasure or funny trinket among the stalls here. After you've had a thorough browse of things, grab something to eat from one of the food vendors.
You'll find this community marketplace in Chartrons. Once you've explored everything you want to at Marche des Quais, check out nearby Aquitaine Museum, Place de la Bourse, and Place des Quinconces.