If you are wondering what to do in Helsinki during your stay, there is a lot of good suggestions on website https://www.myhelsinki.fi/en
Do you like walking? If you want really feel the heartbeat of Helsinki, you can have a walktrpi: walk to the Senate Square, Helsinki Cathedral, the leisure boats at the end of Aleksanterinkatu and by Halkolaituri, the Market Square, Old Market Hall, Esplanade Park and Railway Station.
If you have more time, you might want visit the historical Suomenlinna sea fortress by ferry. Please reserve minimum 2,5 hours for the visit. More information can be found on https://www.myhelsinki.fi/en/see-and-do/sights/suomenlinna-sea-fortress.