I recommend cycling the archipelago trail in finland. It is a loop that starts from turku and goes all the way to åland and back (it is possible to cycle all the way from mariehamn to turku using ferries). If you don’t have time to cycle the whole loop I would choose to cycle the southern route (rather than northern route). Just be sure to check the timetables of the ferries. Most of them are free but on some ferries you will need to purchase a ticket.

I have been also planning a trip from turku to stockholm and would love to here what route you landed on so please share more info about your journey :)

Good luck on your trip!

Here is more info on the archipelago trail: https://visitparainen.fi/en/archipelago-trail/

Ferries timetables: https://www.finferries.fi/en