There are beaches close to the city.

Amsterdam Plage
A man made beach next to the Silodam industrial complex behind Centraal station. (06 46 016005).

Blijburg aan Zee
A beach with an active party scene about 15 minutes by train from Centraal Station.

Bloemendaal aan Zee
A beach with a party atmosphere and night clubs.

NEMO Beach
A beach on top of the NEMO science building and even slants!

Wijk aan Zee
A trendy beach close of Bloemendaal.  

Zandvoort
About 20 minutes train or drive from Haarlem and is well equipped with facilities.