Have you been to Helsinki before and seen the sights there? What are your interests?

Most commonly, in Helsinki people enjoy the harbor/market in the morning, a boat to the fortress island if weather is good, a tram ride around the commercial center, up along the west side of the lake, a visit to the churches, etc. Most tourists find 1 day of activities in the city itself.

A day trip by boat to Tallin is easy with several companies and times to choose among. From the dock you can walk or ride to the old city and spend several hours, including a walking tour. If you search here, you will find several trip reports from people who have stayed longer and toured further in Estonia.

Another popular day trip is to Porvoo to ride on the water one way, see the small area of wooden buildings (with shops and restaurants), and bus the other. (It is small.)

Beyond that are a few destinations such as Turku, depending on your interests.

In general I like seeing some history, wandering/walking around towns/cities, eating/drinking and also having some conversations with locals.

Not too much into musuem's. More the live culture thats a fabric of everyday living for me nice architechture, narrow streets meant for walking etc is a part of culture.

Shriram

Thanks for all the suggestions, ended up taking ferry from Turku to Stockholm, spent day (6 am to 5 pm) then to Tallin (full day) and back to Helsinki on Monday.

All lovely places though wish I could have spent more time in Stockholm. Guess I could have gone to St Petersbourg with ferry (no visa reqs).

Shriram

You should also have visited Tartu in Estonia. A very lovely city in Estonia. I liked it even more than Tallinn. Buses leave hourly from Tallinn coach station there. It is also easy to fly there by British flyBE airline from Helsinki.
Thus my ideal trip would be: Helsinki - a plane to Tartu - bus to Tallinn - speed ferry back to Helsinki or vice versa.