If you can head over to Avyeron, an adjacent department, The Tarn Gorges are very beautiful. It's very dramatic scenery, great drives, walking and activities on the river. Depending on how far you want to travel, you can also go to see the Millau viaduct if that kind of thing interests you. But these might be a too long drive from Cordes since you are only staying 3 or 4 days.
In Tarn, I'd also recommend Albi. The Cathedral is amazing.
What kind of things are you interested in seeing?