Technically not Amsterdam (although sometimes referred to as Amsterdam beach) but I am still going to recommend Zandvoort beach. It is only a short bus ride from Amsterdam city center (bus 80 leaves from Elandsgracht bus station, closeby Waterkant) and this is something I do quite regularly on weekends or even for an evening. At the beach you can find several really nice beach clubs/restaurants. A perfect little getaway from the city.
2041 Zandvoort, Netherlands