Finland is as bicycle-friendly as any country you’ll find, with plenty of paths and few hills. Bikes can be taken on most trains, buses and ferries. Åland is particularly good for cycling. Helmets are recommended but no longer required by law.
You can hire a bike in nearly every Finnish town. Most campgrounds and many urban hotels offer bikes for a small fee or for free, but these are made for cycling around town, not for ambitious road trips. Better bikes are available at dedicated outlets. Expect to pay around €15/90 per day/week for a good-quality road bike and €45/120 for a mountain bike.