Keep an eye out at Halles Centrales — you're likely to uncover some treasure or funny trinket in one of its many stalls. After you've had a thorough browse of things, grab something to eat from one of the nearby food vendors.
This Rennes Centre community marketplace is an ideal spot to start your day of sightseeing. There's plenty to explore in the area, such as Parliament of Brittany, Thabor Botanic Gardens, and Parc Expo Rennes. Wander around at your leisure at Halles Centrales before hitting up these other fine sights.