A particularly interesting one would have been
Seo d'Urgell in Spain. Built to be the airport for Andorra, it sits on the cut-off top of a hill in a fairly narrow valley surrounded by 8-10,000' mountains with a long drop off each end of the runway. Apparently the charter companies took one look at it and said "no thanks". There is a runway, terminal building, customs, car park etc. but it has never been used apart from a few light a/c and sunday markets. Spooky.