Source: Jaan Männik
You cannot visit Viljandi and not go see the lake. Best views are from the castle hill but once you go down there is a very good playground for children, opportunities to rent a boat etc.
the beach is beautiful, but the water quality is poor, i would not go swimming there, even the locals dont
The lake is really big, you can spend hours walking round it. The park in its surrounding is great, thick and beautiful. Really reccommend it.