Helsinki is very close to Tallinn, Estonia (accessible via a 2hr cruise). We highly recommend visiting the old town.
Tickets may be booked either from
https://www.eckeroline.com/
https://www.tallink.com/destinations/estonia/tallinn
Ateneum (art museum)
2 Kaivokatu, Helsinki, Uusimaa 00100, Finland
Oodi (modern library)
4 Töölönlahdenkatu, Helsinki, Uusimaa 00100, Finland