The website Suomen Vesiputoukset has compiled all major waterfalls in Finland into a neat catalog and provide a KML-database as well. The number of waterfalls is rather restricted, so it would make a nice roadtrip for the summer holiday to go and visit some.
You can browse the collection via the map below.
The waterfalls are mostly small, but luckily some of them are easy to access:
I converted the data into an ESRI Shapefile to be more usable in GIS software. You can download the shapefile here: Suomen-Vesiputoukset-19092014_shapefile