During summer I'd highly recommend Turku Archipelago! It's really beautiful over there! Visited it myself few summers ago and would like to go there again.
And if you're already near Turku, step by at the Moomin's World in Naantali as well. I'm sure I enjoyed it as an adult as much as the kids do
Also Keski-Suomi is nice to visit during summer - all these lakes and stuff! <3