Avoid Helsinki at all cost.
Karhunkierros is the standard finnish /out/ experience, so if you don't want to take any risks, that's probably what you want to see. If you want to see tundra and fells (hills), go to Lapland. If you want to see lakes and eskers (squiggly hills), southern, but not too southern Finland is your best bet. For sea and islands, west coast is your best bet, especially near Turku, tho prepare for assholes, especially near Turku.
If you want the best of the best, i'd recommend Repovesi. Kaakkurin kierros is prob the best hike path in the country, and so far hasn't been ruined by tourists like the majority of Lapland has.
Also some links:
https://finlandnaturally.com/national-parks/http://www.matkallakotimaassa.fi/https://kartta.paikkatietoikkuna.fi/?lang=fiBtw. carrying a knife that isn't a puukko will ensure you will be laughed at.
