Start with the view. Pick a pretty day — sun out, slight breeze, temperatures mild — and there are few more pleasant places to while away an hour or two than the downtown Central Library’s Rooftop Terrace. Five stories up, you’re surrounded by skyline. The landmark, brick and brownstone New York Life building looms to the north. To the east, buildings that once housed such swanky shopping establishments as Harzfeld’s and Pecks line Petticoat Lane.