I visited Tallinn as a day trip from Helsinki. It only takes a hour to get there on one of the many hydrofoils. (Don't buy your ticket from the Helsinki tourist information place, as they add a surcharge - just walk down to the harbour and buy them direct!)
The centre of Tallinn is not very big and can be covered easily on foot - there are many interesting buildings and museums to visit - it's worth buying a 'Tallinn Card' that gives you access to all the places of interest, and travel on the trams and buses.
The highlight has to be either, the views from the top of the Oleviste church tower (which in the 13th-century was the tallest building in the world!), or the rather disturbing painting of the Danse Macabre, by Bernt Notke in the St. Nicholas' church.