I went to Tallinn in 2005. It is truly one of the world's most beautiful and largely undiscovered cities - the entire old town is a world heritage site. The old town is surrounded by a medieval wall with numerous large watchtowers. The old town is full of beautiful unspoilt buildings, churches and the superb domed cathedral of Alexander Nevsky. The views from the ramparts of the old town across the city are terrific. It is a very safe city, although it has become a target for drunken hen and stag parties. There are many street cafes in and around the main Town Hall Square.
I would advise a trip to Kadriorg Palace, which is a half-hour walk from the city centre - wonderful gardens and a nice walk through a park down to the seafront.
It is also worth taking the high-speed catamaran 1 hour across the Baltic to Helsinki, which is itself a beautiful city.
Have a great time.