finland: "must see"-places in mid-country lake area (Helsinki-Kajaani)

Hi!

We plan a 3week trip to finlands mid-country lake area in august, and
we really look forward to having 3 weeks of no cities, no stress and
nothing else but nature ;-)

But route planning is a little complicated, as this area is so huge.
It would help a lot to have some points that we can plan our trip
along. Otherwise we could just take a ruler and make a straight line
from Helsinki to Kajaani ;-)

So does anyone know about places along this line (Helsinki-Kajaani)
that are really worth visiting (nature-wise, culture-wise)?

Any suggestions are highly appreciated! Thanks a lot,
negg (Martin Schuster)