During the summer season several 'beaches' open up in the city. Amsterdam plage is one of those and probably the only one in (or at least near) the center. It is located West of central station in the IJ channel.
Besides sand you can find a bar/restaurant and a small fair. Also every now and then a party is thrown.
The water of the IJ is not swimming water, so either hope that they get the pool up or just relax in one of the big bean bags.
As it is not allowed to bring your own drinks and food you have to bring some money, unfortunately prices are quite high.