Rennes has an attractive historic district, but it's a healthy walk from the train station. I'd say at least 15 minutes and quite possibly 20 or a bit more. Other good news: There is a huge construction project (new RR station, I'm told) that has streeets totally blocked around the station and barricades set up that are high enough to block sightlines.
On the other hand, there are attractive-looking restaurant options within 2 or 3 blocks of the RR station (along Ave. Jean Janvier). I was extremely pleased with my moderately-priced dinner at Il Basilico--tuna with accompanying vegetables. The menu is heavy on pizzas, but in my experience pizzas in France just aren't as good as what you can get in Italy.
Incidentally, I went to Fougeres today and really liked it. It was strangely quiet, I thought, but perhaps its tourism is very weekend-oriented, and tomorrow's market day. Also, I did not go inside the castle, but rather spent about 4 hours pounding the pavement. The tourist office's map understates the area with historic architecture.