The Netherlands has a great "beach culture". With so much of the country near the North Sea, it's no wonder that tons of great beach towns have popped up, offering great cafes, bars, and nightclubs along the windy shores.
An easy and accessible beach is Zandvoort, only a 20 minute drive from Amsterdam. With a wide white sand beach, lots of great restaurants, and tons of "beach clubs" (where you can rent wind-screens and lounge chairs), Zandvoort is a popular destination. Even more popular is Bloemendaal beach, which I've mentioned a few times before. Bloemendaal is more catered to younger Dutch locals, usually from Amsterdam. It has great all-night parties, and lots of well-known DJ's are featured there.
Zandvoort is more child-friendly, and is a haven for many tourists from other European countries, such as Germany.
When the city becomes unbearably hot in the summer, many citizens head to the shore to beat the heat and feel a cool breeze. This is all the more necessary in the Netherlands where it is rare to have an airconditioning unit in your home! Last summer, however, it was only really really hot for a few weeks in July, so I can understand the reasoning behind not installing an air-co unit.
Sometimes the traffic from Amsterdam to Zandvoort/Bloemendaal is so thick, that they shut off the beach and prevent anymore people from heading to the shore. Those stuck in the traffic jams often just park roadside, unpack their picnics, and spend the day at some random park or grassy patch.
For those who do make it to the beach, a long day of sun-tanning (the Dutch loooove tanning), voetbal (soccer), windsurfing, and relaxing ensues. With a pint of beer, some great bar snacks (ie. bitterballen) and plushy couch at an outdoor cafe, you can't go wrong with a day-trip to one of the Netherland's great North Shore beaches!