The Pihlajasaari island in front of Helsinki is one of the popular islands, where you can travel by a ferry in ten minutes from Kaivopuisto during summer time. There is also a docking pier for small yachts and we stayed there for the night sleeping in our sailing boat.
Google Maps gives you actually quite nice view of the whole island, or islands.
Here is also a list of all maritime attractions in Helsinki.
A fully licensed cafeteria-restaurant at Pihlajasaari.
Sailing has been our hobby for over ten years and we have been sailing from Espoo to Turku and Åland and Sweden too. Our boat is shared with another family, who has got also three children. Usually both families will sail 2-4 weeks during summer holiday exploring the archipelago in Finland, last summer in Sweden and this summer probably in Estonia. We also do weekend trips to islands near by.
I have also been blogging in Finnish about sailing, but lately I have written more and more about environmental issues like marine protection. Here is the link to it, in case you are interested. http://venelomalla.wordpress.com