Regatta is our choice for the nicest cafe in Helsinki. It is placed in a historic small cottage that only fits few tables and used to be fishermens fishing net warehouse in the 19th century. Regatta is open all year round, every day. In the wintertime a hot chocolate with korvapuusti (cinnamon bun) enjoyed in the hut is a great way to stay warm.
In the summertime there is an outdoor seating as well as the places inside will run out. The refill concept here is pretty interesting, you will get paid back 5c if you refill you coffee cup. You can also hire a canoe or paddleboards here or buy sausages and grill them over the open fire.
Tram #8 will take you the closest with about 900m walk from the nearest stop.