Jarun is Zagreb's big recreational lake, a place where the city loosens its collar. Locals come for swimming, rowing, cycling, skating, bar terraces, and long summer evenings by the water, making it one of the best ways to see Zagreb outside its historic core.
It is most rewarding in warm weather, when the paths and beaches are busy without feeling like the Adriatic coast. Bring a bike or plan a long walk, and stay into evening if you want the lake's bar scene rather than just the daytime park.