Lonely Planet recommends M/S Puijo!

"The double-deck MS Puijo is one of Finland’s most popular and scenic lake ferries, cruising from Kuopio through a complex system of lakes, locks and canals to beautiful Savonlinna, Finland’s summer 'opera town'.
 
The 10-and-a-half-hour cruise passes through some of Finland’s finest Lakeland scenery, where the blue of the lake seems to wash into the sky and is separated only by the green of forested shoreline and hundreds of tiny islands."

Read more: Cruising Finland's Lakes