There are a number of free things, I think most families do. Let ‘s list a few here anyway, just to get started:
1. Picnics in parks: pack picnic mat, snacks, maybe a Thermos with tea/coffee for parents, maybe a ball or a game of mölkky, and head to various parks or beaches for a relaxed but fun time.
2. Free background music: many great concerts are held in Helsinki in places, where you can hear the music fairly well outside the concert area. With friends, drinks, food and sun the evening will be great!
3. Little trips within the city to different playgrounds (Kaivopuisto has a nice one), maybe a harbour to look at sailing boats (have you been to Herttoniemi?), some bay or beach with ducks (Töölön lahti) or a market place. Doesn’t really have to be that special as the trip itself is fun. Children are so good at getting excited! It will be more exciting if you go by metro, train or tram or whatever you normally don’t use. In Helsinki public transport is free, if you have a stroller. Take your time and look at things e.g. if you pass a building site. or turn it into a treasure hunt with picking flowers or stones… Good way to explore the your home town as well.