Tallinn (see map) makes a nice complement to ultra-modern Helsinki. Helsinki has less history than Boston. Tallinn, on the other hand, has a beautifully preserved medieval core.
In medieval times, Tallinn was two separate cities. One city used to be a second-tier Hanseatic trading town filled with Swedes and Germans. This one here is a typical Hanseatic merchant house:
The other city used to be more aristocratic and Russian. The aristocrats' city was on higher ground and had a wall around it to keep the riff-raff out.
Tsar Peter even built a holiday home here in Tallinn. The Tsars would come here from St. Petersburg for fresh air.