Tallinn is just a couple hour's ferry ride from Helsinki, so it makes a good day trip from there. An afternoon there is about enough to wander around the old town area a bit, visit some churches and antique shops, and see some nice views.
The first thing we encountered on our way into town from the harbor was a little odd. Several hundred people in a field appeared to be getting taught a Macarena-like dance by a guy on stage with a microphone. We couldn't understand what he was saying, unfortunately, except for when he counted them off 1-2-3-4 in Estonian. No idea what that was all about. Once we made it into the Town Hall Square, we found a military band playing tunes like Something Stupid, and Puttin' on the Ritz. They wrapped up soon after we got there, though (Finale: The Can Can), and we didn't get to dance to them. (Cobblestones, crowds, and backpacks would have made that hard, anyway.)
Near the square we met a guy selling miniature kites, which are great because you can just stroll along the street on a breezy day, with your little kite skipping along ahead of you like a dog on a leash. We got a couple of those, though I haven't tried mine out yet. Someone else who tried to sell us stuff had a collection of old Russian passports and communist cards. We passed on those.
There are several interesting churches to visit, though you can't necessarily take pictures in them. St. Nicholas' is now a museum and also has a collection of church bells from around Estonia. Another, whose name I've forgotten, treated us to a few minutes of organ music. I think someone was just testing it out or something. A lot of the other places have sort of blended together in my mind.
Our ferry back to Helsinki was canceled on account of drizzle, so we had to stick around another two hours, which gave us time to go back to St. Olaf's Church. St. Olaf's has what was once the highest spire in Scandinavia, and if you climb to the top of the tower, you can get the best views of Tallinn anywhere. The tiny stone spiral staircase has 233 steps going up and 234 going down (I counted). Must be magic. Once you've climbed those, you get a 23-rung ladder to actually reach the viewing platform. From there you can see the churches and towers of old Tallinn contrasting with the skyscrapers and office buildings of the modern part of the city beyond. It's a great way to end a day there, both because you can pick out places you've already been, and because you're legs will be ready for resting on the ferry when you're done with the climb.