Katajanokanluoto is a charming small island (95 x 65 meters) in front of Helsinki mainland. Earlier it was the island of maritime pilots and they used the still remaining wooden house which was built in 1876. Nowadays you find Cafe Kobben there. Read separate Tripadvisor reviews about Cafe Kobben which actually is a pizza restaurant.
It is easy to get to Katajanokanluoto in 7 minutes from the city center of Helsinki. Just hop aboard M/S Julia (Meritullintori Pier, not the market square). You can buy round trip tickets online (7.50 euros) or on the ship (8.50 euros). This year they are going to operate at least on Wednesdays, Fridays and Saturdays (weather permitting) until August 24th. Departures are once an hour between 11.15 a.m. and 4.15 p.m. and return transfers start every hour (on the hour). However, it is a good idea to check timetables.
If you only want to see the charming island, 30 minutes is enough there. If you are going to enjoy pizza, you need more time. Also rental SUP boards may be available there and they have a tent sauna and a glamping tent, too. It is important to check in advance if these services are available. You may also rent the whole island for private events.
The island is very small and it may be very windy there. However, I loved it very much. It was extremely relaxing to sit on the rock and watch the sea and birds. This is like a traditional Finnish countryside cottage. In my opinion, everybody should absolutely see it.