Your favourite day trips out of Lyon?

Vienne, Valence, Lyon.

Lyon itself has enough for three days. Vienne if you are a big enthusiast for Ancient Rome. Perouges is a medieval walled town in the region that can easily be reached from Lyon by car or bus. Not sure if it counts as a "day trip" as it's served by the Lyon local bus system, but there's a nice car museum at Rochetaillee north of the city, if you like old cars.

Philip mentioned Perouges and I heartily second his recommendation. It is gorgeous. A quick google and you will see what I mean. Your camera will get a work-out.