There are a number of different reactors in use, most commonly water-cooled pressurised or boiling water reactors - though other reactor types are kept cool with the help of liquid metal, gas or molten salt.
My recommendations would be Christiania, the swimming area at Islands Brygge (where you can take a dip in the cool, clean salt-water of the Copenhagen channel), or rent a bike and head north along the coast towards the beaches in Klampenborg and beyond, while looking at the magnificent houses along the way.