Lakes and riversides
Vodní nádrž Džbán
A large reservoir surrounded by grassy beaches. Watch out for the naked section. An added bonus is a huge ramp which snowboarders can jump from in to the lake. The ramp is hosed down and then, whoosh, off you fly into the lake. Whether you participate or just watch, it's very entertaining. Take the metro to Dejvická (line A), then take tram 20 or 26 to Džbán, and follow the crowds around the Citibank-building.
Hostivař
A little further out, but they have a brand new volleyball court for rent, a sandy beach, and a treacherous water slide. Take tram 22 or 26 to Hostivařská or the metro to Háje (link C). You have two choices - weave through a fantastic woody park or meander through paneláks - but taking the metro to Háje is the quickest route. Exit the metro, take a right and walk to the end of the long open mall. Take a left, and walk through the large apartment complex. Cross the main street called Výstavní and walk down a small residential street. You will then see the lake.
RIVERSIDE BEACHES
Go Žluté lázně
On the east bank of the Vltava, in Prague 4 - Podolí, this riverside recreational area offers a large beach for bathers unafraid of taking a dip in the river, plus a children's pool.
Other attractions include beach volleyball courts, a three-on-three football cage, a climbing wall, table-tennis tables, and a pétanque area. There's also a restaurant and beer garden, a nudist beach, frequent DJ sets and WiFi hotspots throughout.