Opentable is a good place to start, though availability, even on opentable, is very limited (and only a partial 'inventory' is released).

Hotel guest do have the priority for getting up to the roof, and I've found that sometimes once there it is possible to ask for table. Platinum members also have this benefit (even if not a hotel guest).

I've found that this is a great place for happy hour/early drinks and light food. If you're looking for something on the slightly more substantial side, the chicken sandwich is great. As for drinks, classics like a Moscow Mule, Dark and Stormy, or Mojito are excellent. If you don't mind a pink drink, try the strawberry lemonade.