If you visit Helsinki during a spring, summer or fall, this is a place to pay a visit!
Take a taxiboat from Merisatama to Uunisaari and walk via the bridge to this lovely island occupied by one of the local yacht clubs. Enjoy your afternoon drink here. Inside or at the terrace, -it does not matter = the view is still one of the best in Helsinki.
If you get enchanted, stay for the dinner. Though it is a not a bad idea to take the boat back and head down town, since the food is nothing special and surely over priced.
A great place to organice parties and weddings, if that is your aim :)