Zandvoort is the seaside resort on the North see just about 20 minutes by bus (or by car) from Haarlem. You can find there lots of nice restaurants, hotels and casinos. During the summer season Zandvoort is overrun by thousands of tourists and local people who enjoy all the advances beach can offer, especially all-night parties and open-sky concerts. Concerning swimming, some tourists who visited for example Croatian or Greece cost or islands, might be a bit dissapointed because the water seems not to be such crystal and colorful as it is there, but blue flag I noticed on several locations and local people convinced me that water is clean and suitable for swimming.
How to get there:
Zandvoort is connected by bus with the centre of the Haarlem (I forgot the number) which runs approx. every hour. Do not try with your own car because finding a parking place could be a long-lasting and exhausting adventure.