Tallinn is one of Europe's best preserved medieval cities, and although the old town is not very big, it does make for a day or two of exploring and strolling its ancient alleys and streets. Being a former Hanseatic city, its architecture closely resembles its counterparts in northern Germany, but with a Baltic and, yes, Russian, flair. The hill with the parliament building and the Alexander Nevsky Cathedral is very nice, and there is a church with a huge belfry (St.Olav's, I believe) that you can climb and that gives you a great view of Tallinn and its port (it was the world's tallest building in the 16th century, I've heard). A very nice destination, but probably not enough for more than a couple of days. It should be combined with Helsinki (very easy), Riga, or maybe St.Petersburg.