we just return from Helsinki and Tallinn.
you just walk over to the market square. There you find any kind of sightseeing tours:
by boat (14-16 EUR)
by tram (6 EUR/daypass Take Line 3 in any direction as it works like a "8" through Helsinki - depending to the quay it could be that you need to connect with tram 4 to Market Square area
by bus (hop on-hop off)
Although you can walk yourself we had a personal guide for 50 EUR for three hours who met at the pier and went with us through Tallinns old town. BREATHTAKING!!!
Have a nice stay!