Tallinn's Old Town (note the capital letters) is a jaw dropping experience. If you leave Estonia without walking around the Old Town for at least an hour or two then you should go home with your head bowed in shame. This is World Heritage in its condensed form. The whole of Old Town can been seen in a few hours (or a few decades depending on your schedule). For a focused tour I suggest Pikk tanav (Long street), one of Tallinn's oldest boulevards. Start from the top of Pikk Jalg (Long Leg) in Toompea. Behind you should be the Estonian Parliament building (big and pink) and Nevski Cathedral (big and Russian Orthodox). Head down the Hill to Pikk Jalg Gate. This gate and the wall to your left (know as the Wall of Mistrust) kept the Nobility of the upper town safely separated from the riff-raff downtown. From this gate to the far side of Old Town is less than a kilometer and includes loads of famous buildings as well as enough cafes, restaurants, bars, souvenir shops and museums to satisfy any reasonable cruise ship passenger.
I have lived in Tallinn for ten years now, and yet, every time I visit the Old Town I find myself well amazed at its beauty, cultural significance and incredible luck in having survied as wonderfully as it has. Don't miss it!