4 days Helsinki and a day trip to Tallinn that was the trip that I made in late August 2012.
Helsinki is modern city with some nice old buildings aswell like the Uspenski Cathedral or the Lutheran church. Besides taking a ferry to Tallinn it’s also possible to take a ferry to some other Islands and I took the one going to the Suomenlinna Fortress which is a Unesco World Heritage site. Tourists but also locals took the ferry visiting the Island and it’s fortress. I walked around for about 2 hours following a path that goes around the Island, stopped in a small museum on my way. It’s nice that you can switch from a city to an Island and back when you in Helsinki. You can see the city from the Island as it is so close, so if you are a local you don’t have to go very far to be in a total different place.