Island hopping along the Greater Archipelago Ring Road.
There is also a shorter version which bypasses the less populated outer islands, as the long one is a bit tight to fit into a single day. The ferry Houtskar-Inio will probably be the critical one with fewer round trips/day and a rather early final round trip, but check the timetables. You'd have to leave early in the morning to avoid being pressed for time, or indeed plan to stay a night in the archipelago. The main ferries run on a schedule, the road ferries in between according to demand. Leave spare time for delays.
Access roads both from Kaarina via Nauvo, or Raisio via Taivassalo are rather busy A-roads with low speed limits and a fair bit of surveillance. Some sights on either side o the "real deal", e.g. Louhisaari manor or the ruins of the Bishops` castle. Expect ferry queues on weekends on the Nauvo side, to an extent also in Heponiemi. Bigger service stations are in Nauvo village or in Taivassalo, 95-octane fuel is available at the K-market in Houtskär or at some marinas in a pinch, probably.
Narrower B-roads on the islands, a few blind tight corners, surprising road junctions, pavement a bit rough in places (possibly patches of gravel to catch an unwary biker), several road ferries in between. Expect touring cyclists, slow-moving farm machinery, so don`t overdo it even if the roads permit a burst of speed from time to time. Life has a slower pace there. You`ll get a glimpse of the Baltic from time to time, but most of the route is inlands thru fields and forests. You`ll find a viewing tower next to the Mossala-Iniö ferry (camping, restaurant) with a beautiful panorama of the islands, and there are some nice places to visit, e.g. the old wood church in Houtskar village or the stone church in Norrby on Iniö (there is also a bigger beautiful one in Nauvo and in Korpo village). A number of places to stay, if you`d like to spend a night in the archipelago, but book ahead. You might also want to hike the trail up the Kasberget hill on Iniö for a splendid view. The village shop (founded in 1921) at Norrby on Iniö is fun, or visit the family-run K-market in Houtskar village.
Summer weather out in the archipelago is often fair and sunny even if there are billowing clouds and rain showers on the mainland. When under a solid high pressure system, you may even catch one of the hotter (25+ C) calm days, while there is a sea breeze on most summer afternoons. Sunlight is quite intense, you`ll need more SPF than you thought.
This is part of the ring road of Archipelago (Finland).
Very pleasant ride...