I don't sleep at all on aircraft (well, not in Y anyway!) but 3 days in New York would sound perfectly reasonable to me (I'm UK based). As it happens, I'm off to Denver for a 4 day weekend next month, 2 days of which will be spent in the Rockies doing a *lot* of walking and the rest shopping and eating round Boulder. Can't wait. Admittedly we're in J this time though (and the wife is very excited, it's her first time in business class). Paris for 3 days sounds fun, is about the right length of time. Go for it.