If you plan to visit many museums in Finland, then think about buying the Museum Card. It costs about 86 EUR, and gives you access to over 360 Museums in Finland, of which 8 are located in Aland.
More details regarding the Museumcard, Museums in Finland and Museum Events here.
museokortti.fi
Finnish Museums also offer certain days during the month or year where not entry must be paid. Check the website and plan your trip well.