Kungsbacka Zoo is a small family animal park outside Porvoo, focused more on approachable domestic and farm animals than on big-ticket wildlife. For travellers with children, that can be exactly the point: goats, ponies, rabbits, and other familiar animals make for an easy rural outing.
It is not essential Porvoo, and adults without children may prefer the old town or riverfront. But as a half-day family stop, especially in summer, it fills a useful niche. Confirm seasonal hours before going, since small Finnish attractions often operate on a limited calendar.