If you are only staying a day, then you are really looking for a city that has direct flights from NYC and to Naples, that will thin out the list quick. Obviously, Paris, London Amsterdam, Frankfurt and other major hubs are candidates, but NYC may have direct flights to many cities in Europe not available to other markets.
One resource you can look at is flightconnections.com. Drawback, besides the ads, is that it really looks at "where can you fly direct from Naples (by putting NAP in the departure box and leaving the "to" box empty). A map will show all of the routes, you can click on a route, and it will tell you the Airline, and if Seasonal, the months it flies, and nice for the Budget airlines, the days they fly. Naples shows some 50 direct destinations, but usually those routes go both ways. You can then go to that Airlines website and look at flights. The other caveat, I am not sure how much this site is affected by Covid related route changes, if it is showing scheduled routes, regardless if they are flying, and how routes may change with recovery.
Just browsing...Madrid, Barcelona, Lisbon, Prague, Berlin, Krakow, and many "non-major-hub" cities pop up, so probably some good options for NYC. Myself, given just one day, I would probably choose a place I have been to before, just for ease and comfort. Visit some old haunts, walk, and not feel I need to "do sights".