What's the must-see thing in Helsinki?

Temppeliaukio Church in Töölö, not Helsinki Cathedral. The cathedral is the postcard, the first thing visible from Helsinki's South Harbor, and it's fine. But it's a white neoclassical Lutheran church with bare walls and wooden pews. You've seen one like it in Stockholm or Copenhagen. Temppeliaukio is the building you haven't seen anywhere else. Architects Timo and Tuomo Suomalainen carved it directly into the granite bedrock of a hill on Lutherinkatu 3, and it opened in 1969. The walls are raw, unfinished rock face with the drill marks from excavation still visible. Copper coils spiral up to a shallow dome that lets in diffused natural light even under Helsinki's frequent overcast skies. Sounds don't bounce off the 8 metres of rough granite the way they bounce off marble in a conventional church. Conversations at normal volume disappear into the stone within a few metres of the speaker. Entry is €4, about $4.65 at current rates. Fifteen minutes is enough to take in the full space. Go before 10am on weekdays. By noon in summer, the line of tour groups wraps around the block on Fredrikinkatu.

Senate Square is the city's center of gravity, and you should still see it. Carl Ludwig Engel designed both the square and Helsinki Cathedral as a single neoclassical composition between 1818 and 1852. The white exterior of the cathedral sits at the top of a steep granite staircase that faces due south. On clear June evenings the sun doesn't set until after 11pm, and those steps fill with people sitting with takeaway coffee from Fazer on Kluuvikatu, two blocks east. The interior is almost aggressively plain. No gilding, no frescoes, no side chapels. Free entry, no reservation needed. While you're at Senate Square, walk 600 metres east to Katajanokka. Uspenski Cathedral, the red-brick Russian Orthodox church completed in 1868, sits on a hill above the harbor. The contrast between the two cathedrals, one white Lutheran and one red Orthodox, built 16 years apart, tells you more about Finland's position between Sweden and Russia than any museum panel will.

Suomenlinna is the third pick and the one that needs half a day. The fortress covers six islands, 15 minutes by HSL ferry from Market Square at Kauppatori. Ferries run every 20 minutes in summer, and the fare is a standard HSL ticket at about €2.80 on the app, or covered by any day pass. Sweden started building it in 1748 as a naval defense against Russia. Finland inherited it after independence in 1917. UNESCO listed it in 1991. The tunnels smell like damp granite and seawater. Even in June, bring a jacket. The wind off the Baltic drops the felt temperature well below the 18-19°C you'll have on the mainland, and there's no shelter once you're out on the ramparts. The Suomenlinna Museum near the main quay covers the military history in about 30 minutes. After that, head for the Kuninkaanportti on the southern shore and the dry dock from the 1760s. Everything else you'll find by following the blue-marked coastal path between them.

A few things on every Helsinki list that shouldn't be your priority on a first visit. The Olympic Stadium from 1938 is a stadium. Unless you want the 72-metre tower view for €8, it's not worth the tram ride. Kiasma, the contemporary art museum Steven Holl designed on Mannerheimintie, rotates exhibitions that tend to need 2 hours or more. If you want one art museum, the Ateneum at Kaivokatu 2, open since 1887, holds the largest collection of Finnish painting, including Akseli Gallen-Kallela's 1891 Aino triptych. Entry is €18. The National Museum of Finland on Mannerheimintie, open since 1910, covers Finnish history from prehistory forward, but the Jugendstil building itself is more interesting than most of what's inside. Mind you, if the weather turns to rain, any of these fill a couple of hours well. With 2 clear days in Helsinki, spend them on Temppeliaukio, Senate Square, and Suomenlinna.