I'd second Peter337's suggestion of La Rochelle - although it is probably on the outer limit of the distance you are looking at.
An alternative might be Chartres - busy club airstrip at the weekends and a fairly walkable town.
At a similar range Laon - again a busy club airstrip but the old town perched on top of that rock outcrop can be seen from 40 odd miles on a clear day. Variety of WW1 remains etc in the local area and plenty of hotel accomodation in the commercial part fot he town.