Free self-guided itinerary. This is a travel guide, not a bookable guided tour.
Unlike most European capitals, Helsinki’s centre can be seen properly in a day. It is compact, flat and well connected, and this guide is built to use that rather than to cram.
Morning. The designed centre on foot, while it is quiet.
Midday. The market and harbour edge, and lunch where people actually eat rather than where the view is.
Afternoon. Either an island by public ferry, or the design and architecture district. Both are good; doing both is a rush.
Evening. The city is small enough that you can walk back through the centre and see it in different light.
Treating the island ferries as a separate excursion when they are ordinary public transport. Underdressing for wind. And planning a long outdoor day in December, when the light disappears mid-afternoon.