Kasinonsaaret are small islands close to the centre of Savonlinna, used by locals for an easy dose of lake nature. Paths thread through the trees, and in warm weather the islands are a relaxed place for swimming, picnics, and barbecues.
They are not a headline sight like Olavinlinna, but they are useful if you want a low-key walk by the water without leaving town. Bring what you need with you and treat it as a simple outdoor break.