I really enjoyed the old town in Tallinn, a city I overlooked for years. It's beautiful and charming place with great character and local culture that deserves to be seen.
Tallinn combines centuries of history with high tech modernity. You can enjoy the best of both worlds while sampling food at a steadily increasing number of fine food restaurants.
A must see if you are visiting Helsinki as it's a quick ferry ride away. This city is one of the most beautiful in Europe (inside the old city walls). While there do need to be some kinks worked out in terms of addressing the large influx of tourist, I would imagine in the years to come, this city will explode with greatness.
Whole day trip by ferry (2 hours ferry /direction) , very nice old Hansa city