Podoli is part of Prague with the best (and the biggest) swimming pool. There were tens of cars parked everywhere and trams were full of *going to* and *going from* swimmers of every age.
And there is more! Zlute lazne ("Yellow baths") is a place at the Vltava river with a beach and all you can have near a typical tourists driven beach (they have a little ski slope in winter here, too :). Zlute lazne is very trendy. They must have great marketing/PR team because they had a tv reality show here last year, all of the fashion magazines do their summer shooting here.
And I love that inflatable thing the girl has. Is it a centipede? :)