Prague may be a city without a sea but we have a river and there are many islands here. One of them is called Štvanice and there’s a pumptrack and a water-power station (at the uglier end) and a really nice skatepark (at the nicer one).
Go by tram (no. 14) to a tram stop called Štvanice and then head to the greener part of the island. After about 500m, you are in a skaters’ and bikers’ paradise. There is an annual Mystic Skate Cup held in this skatepark in Summer but if you want to chill anytime, it’s open for everybody for the whole year. You can just sit there with a beer or two, watch the stunts of the skate masters or take a ride yourself.
And if you go even further from the tram stop, you can relax in the grass at the end of the Štvanice island and watch the Vltava river and ducks and boats and ferries… and when you’re relaxed enough, you can leave the island by the ferry you’ve just watched either to Karlín or to Holešovice.