Between Montargis and Châtillon-Coligny there is a really impressive Roman theatre (I have marked it here). We drove past in on one of our expeditions looking at cars.
Every time we go out in the car we see signs pointing to really interesting things we never knew existed. In a way, this is a bonus: we wouldn't travel 300km just to see this Roman amphitheatre, but having it appear out of the blue provides an opportunity to stop the car and have a little walk. If it isn't Roman remains or an abbey it's a prehistoric monument or interesting and unusual church.
besides it being on lists of Roman Theatres. There is some
information on notice boards besides the (locked) gate