We are making a Scandinavian jaunt next month and will be spending a few days in Tallinn. I'm pretty excited about it as it looks like a bit more authentic than Prague (also after spending the week in Norway and Denmark, I'll be looking forward to the Estonia prices).
Our agenda consists of a lot of just walking around old town: Gardens, cathedrals, the wall. We will also spend a day east of town, seeing the sights of Kadriorg Park and going to Pirita beach.
It's also a very easy ferry over to Helsinki, so we're spending a day over there before we fly out of the Helsinki airport.