Torikahvit, having coffee in a market place, is not only my personal summer tradition but a national one, and everyone should try it to really experience the Finnish summer.
My favorite is Hakaniemen tori, which is less touristy than Kauppatori in the city center. But Kauppatori, too, has some really good possumunkki, the pig-shaped (kind of) doughnuts, and that’s what you want to go for: possumunkki and coffee. The cafe stands usually have some combined offers of these together. Also the small market places of Töölö and Punavuori have some cafe stalls.
And if I’m on the go early enough, it is also a nice option to have my morning porridge in the market place: the cafe stalls offer different kinds of porridge every morning. I love drinking my coffee and watching the people going about and shopping. Often there is also a small band in Hakaniemen tori playing some nostalgic Finnish iskelmä classics. After having my coffee, it’s nice to go shopping for some fresh fruits, berries and vegetables from the sellers.