Mariestad is in Sweden.
The biggest freshwater lake in Europe, nice boats, and supposedly a lot of Swedish tourists in summer.
Go swimming, rent a bike, or have some other fun. On Saturdays, there's a market at the main square
There's a fine coffeeshop with great blueberry-vanilla sauce cakes, right at the main street. The Pizzaria can not really be recommended, but it's cheap and ok food
Defenitely sleep at the youth hostel. It's cheap, it's fun. And it has a truly nice setting close to the lake. Moreover, it's several great old wooden houses grouped together in a nice courtyard! If you don't stay there, have a look at it! .