I guess Helsinki as the capital, there a quite a bit of cool looking buildings there.
Lapland, cause of the Santa´s village and northern lights. And reindeer´s.
If you like castles, there is Suomenlinna in Southern Finland and Olavinlinna in Lappeenranta.
You could visit the Moomins village in Naantali.
There are quite a bit of national parks here too, which might be worth checking out.
Maybe better to visit in winter rather than summer, since Finland looks better in winter than any other season. Then again some places look good in the summer too (because of trees and plants flourishing). If the temperature is nice and not too warm.
This seems like a good site for more info:
http://www.visitfinland.com