Visiting Finland in three days (Helsinki)

Dear all,

I will be in Finland for three days at the end of this month, anyone could advise me the things I should absolutely do? I've heard Helsinki is nice but small.

I've also heard about the archipelago next to Turku. But maybe three days is not long enough to do both Helsinki and the archipelago?

Any other interesting place you could advise me for hiking? Or for partying?
Thanks :)