If you want to tryst right (i.e., wrapped in 200-thread-count sheets), you’ll need to plan ahead. At the Four Seasons Hotel, for example, you can’t even check in until 3 P.M., provided there’s a room (starting at $625 a night) available. Book a room for the night before, and request a late checkout so that you can linger until about 2:30 P.M. If you stay past 3 P.M., you’ll be charged an additional half-day rate. The same time restrictions apply at the big midtown hotels, but more moderate prices ($300 to $400 a night) and the number of rooms (2,052 at Marriott Marquis, 2,042 at New York Hilton, 1,750 at Sheraton) greatly increase your chances of anonymity and of finding a vacancy in the heat of the moment. And then there’s the “short-term stay.” On the edge of the meatpacking district stands the Liberty Inn, an otherwise nondescript brick building, where 30 rooms, with Formica furniture and mirrors on the ceiling, reek of bleach (perhaps a good thing in this case) and rent for about $52 for two hours. According to the manager (who is very proud of the fact that a CNN report found the Liberty’s bedspreads among the cleanest in the city), there are always vacancies at lunchtime.