4 days Helsinki and a day trip to Tallinn that was the trip that I made in late August 2012. It was my only trip in the summer vacation of that year.
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.