Suomenlinna is an old sea fortress spread across 6 tiny islands in Helsinki, Finland. A quick 15 minutes ferry ride from the heart of the city, it's an ideal afternoon excursion, particularly on a warm summer day. You can spend hours simply wandering the islands, stopping in the local galleries or taking a picnic break. If you're feeling a bit more adventurous, you can explore the fortress ruins and walk through the various connecting tunnels.
Suomenlinna is a perfect example of why Nordic cities like Helsinki, Stockholm and Copenhagen consistently appear on the top of every "livability" ranking. There are very few places in the world where nature, history and urban development are so seamlessly intertwined.
Admission to Suomenlinna is free, but some museums on the islands have entrance fees. Ferries run from Kauppatori to Suomenlinna approximately every 20 minutes and cost 5 EUR round trip. See the HSL site for full timetable. You can also get more ideas for your next trip to Helsinki by checking out the city's new Places page, which will frequently be updated time with new recommendations!