Most of Helsinki's major cultural buildings -- the opera house, concert hall, and national museum -- are within a short distance of each other around the perimeter of the inlet from the sea called Töölönlahti. The inlet itself is lovely in all seasons, and the walking and biking paths are well trodden by locals. The winding streets just east of Mannerheimintie enfold the Temppeliaukio Kirkko (Temple Square Church), whose unexceptional facade gives way to an amazing cavernous interior. Also nearby, the Sibelius park cuts a large swath out of the neighborhood and borders the sea.
Begin on Mannerheimintie by the equestrian statue of Gustaf Mannerheim, directly behind the main post office and next to the Museum of Contemporary Art (Kiasma). Walk northwest on Mannerheimintie, passing the red-granite Eduskuntatalo on your way to the Suomen Kansallismuseo, Finland's national museum, on the left. When you leave the museum, cross Mannerheimintie then follow the road a short distance north to the Finlandiatalo, the Alvar Aalto-designed concert and congress hall. Behind the hall lies the inlet bay of Töölönlahti. If you walk along the well-used paths that follow the contour of the lake, you'll soon come to the Suomen Kansallisooppera, Helsinki's opera house. From here, you can take the lengthy walk to the Seurasaaran Ulkomuseo.
From the opera house, walk southeast on Mannerheimintie until you see the white tower of the Olympiastadion on your left. Return to Mannerheimintie, crossing over to Cygnaeuksenkatu; take a left on Nervanderinkatu, where you'll reach the Helsingin Taidehalli, with its fine collection of Finnish art. Go a few steps farther and take the small street directly across from the art hall to the modern Temppeliaukio Kirkko, a church carved into rock outcrops. If you still have energy, cross Runeberginkatu and walk west on Simonkatu until you reach Mechelininkatu. Walk a ways north on Mechelininkatu, and Sibeliusken Puisto will appear on your left. Here you'll find the reason you came: the magnificent Sibelius-Monumentti.
Allow 45 minutes to follow this tour as far as the Temppeliaukio Kirkko, adding half an hour for each museum if you decide to venture inside; from the church, it's a 20-minute walk to the Sibelius Puisto. Be sure to check the Temppeliaukio Kirkko's hours, which are slightly irregular. Note that the Suomen Kansallismuseo is closed Monday.