Tallinn is a feasible day trip from Helsinki, with ferries taking only 2 hours one way to dock close to the medieval old town. The city's name is short form for "Taani Linnus", Estonian for "Danish stronghold". The Danish sold the land to Germany in the 14th century, and became a powerful Hanseatic port. This period of glory is reflected in today's old town, which is small enough to enjoy by foot.
On my first trip to Helsinki in 2009, I was curious to spend a day in another country, similar to my day trip to Dublin from London by plane a few years prior. This one ended up being far more relaxing, with great weather offering a nice backdrop to the historic buildings. This is a well-touristed route, and quite hassle-free even for a novice tourist to Estonia. So if you're in Helsinki for a few days, do budget at least one in Tallinn.
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