Several museums offer free entry on the international museum day, for example:

* Åland cultural history museum and art museum at 10-18.
* Kastelholm Castle at 10-17
* Bomarsund visitor centre at 10-17

More information: https://book.visitaland.com/en/book/to-do/2196070/internationella_museidagen_den_18e_maj_/showdetails