Only two stops by metro from downtown (a 0.07 euro ride) is Hydropark island with miles and miles of clean beaches. Yes, you can swim in the river's water and if you think it is too cold, cool down with a beer.
It also has a huge open air gym, dozens of food and drink outlets, a bungee jump thing, beach volley courts, you can waterski, wakeboard and there are lots of old skool playground rides and two swanky clubs that don't close until early morning.
Metro: Hydropark
Updates
This is a good place to go to if you are looking for some (somewhat shocking) local life. However it can be a dangerous place, particularly at night, for foreigners. So only go if you have a local friend and if you are adventurous.
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