One of my favourite places in Helsinki is Töölönlahden puisto. It’s not a particularly large park, but what I like is the “lake” (an inland bay) which is actually a part of the baltic sea. Töölönlahti and the park are only about a 10 minute walk away from the main station, or easily accessible by trams. There is a 2.2 km walking path around the lake which I highly recommend you take! From the western side you have a beautiful view over the lake and some old wooden villas that overlook the lake, and you can also see the white cathedral from here (for me personally still the most beautiful and majestic building in the city).
The actual spot I wanted to get to, is a small outdoor café on the other side of the bay, the eastern side, which overlooks the Opera and the Finlandia Hall. It takes you about 20 minutes to walk to Sinisen huvilan kahvila around the lake from the tram stop, and if you follow the footpath you can not miss it. It’s only open during the warmer months, and then also weather dependent, so it’s a bit of a hit and miss, but usually you’ll “feel” when it’s open. You can take a break from your walk, get a nice cup of coffee and enjoy the great view on the lake.