Finnair has created a website called ‘Stopover in Finland’ that allows you to book a Finnair stopover in Helsinki, for up to 5 nights, on your way to or from your final destination.
The website also provides info on what to do during your stopover stay.
http://www.finnair.com/int/gb/stopover
Photo of Rovaniemi, Finland by Marilyn Jones/ TravelGumbo