Rennes is a university city with a strong food culture — crêperies, cider bars, and bustling brasseries dominate the old town, with a scattering of excellent restaurants for more serious eating. Rue Saint-Malo and the streets around Place des Lices are the densest concentrations of restaurants; Rue Saint-Georges has several good crêperies.
Breton crêpes are everywhere, and rightly so. A buckwheat galette (savoury) filled with andouille sausage, emmental, and egg is a complete meal. For seafood, the Saturday market at Place des Lices gives you excellent raw materials and a sense of what the coast produces. Cider is the traditional drink with crêpes; locally produced cidre brut from around Fouesnant and Plougastel is worth seeking out.