I wrote earlier about Zakrzówek. In day like this, I am of the view that you must to go there!
Judging from the fact that Krakow is very loud city and no one likes spend its spare time there, you should take your bags with blanket and healthy food (don't forget to take water because around the Zakrzówek you will not find any shops) and catch every warm moments with your friends or lover. Please remember you cannot swim in water if you don't have a permission to dive. Only professional divers are allowed to be inside water. The point is that there is a risk you will die or something like this. Nowithstanding, you can enjoy the view instead of swimming!
There is a legend that says long time ago Mr Twardowski was a teacher of magic. One day something went wrong during the lessons and somehow everything burned. Due to the exploision of laboratory were created stones which are named Stones of Twardowski. Beautiful story, isn't it?
Anyway, the place is amazing!
As far as I know, there is a fence along whole body of water. In summer time, young people are very stupid and not resposible for what they've done, because they drink a lot of alcohol during a party in fresh air and want to dive in water. So stupid! Thus you can lie on the grass. The place is located few kilometres from the centre of the city. Oh, thanks to God, without noise :)