Well, Finland in the Summer is just great! I go there almost every year because I prefer not so warm climates.
To stay in Helsinki is by itself a great experience. People on the street, jazz in some corners, great alternative design shops, second hand clothes or music stores. The night life is of high quality. Great concerts all the time, nice operas and theatre. Also some latin bars have fantastic spots to dance salsa, rumba or other latin dances.
A must is to check out the Finnish Tango! it is a must! If you look closely, every film by Kaurismäki has one tango sung! Marvellous!
To visit a lake would be a must. Lappeenranta is a good destination. Close to it there are a few beautiful places. Some influences from Russian culture (border) are present and it can be interesting.
Another spot for lakes is Jyväskylä. Many lakes around too... but, in any case be aware of mosquitoes. Take an insect repellant for your skin and some anti-hystaminics with you.
I hope these tips help a bit... I let you know specific places (bars, old cinemas, alternative shops, bookstores) if you need.
Greetings from Barbara about 9 years ago