Sint Anna beach, Antwerp
A very popular place. I was there on the first sunny Sunday of 2004 and already there where people at the beach. Must be very crowed when it is really hot.
It is a good place to get a tan or for watching the people while enjoying your beer.
Swimming is not allowed.
Take the pedestrian tunnel (you may also take it on bike) under the Schelde from st.Jans Vliet to the left bank. Faced toward the river, when you go to your left, you'll reach the St.Anna-beach (which really isn't much of a beach, and I wouldn't swim in the Schelde either); to your right you can go to the Galgenweel, a lake (or pond? I don't really know).
You can walk further downstream towards the yacht harbour and relax at the St. Anna beach. In front of the tunnel, you'll find this sign, telling you how far to go to every place.