The Sky Lounge is the name of the bar/restaurant on top of the Double Tree Hotel just right of the central train station. No need to be a guest you can just go in and usually find a seat and you’ll be treated to a wonderful view of the city.
Pro tip: the coffee is €6 euro but what they’ll never tell you is that you get a free refill! Beer and other items are on par with other typically tourist haunts in Amsterdam, meaning not cheap for reasonable. Plenty of little snacks to enjoy.