One of the best places I have paddled in Finland is Oulanka river which is quite popular among paddlers. The most suitable part of the river for beginners and those who like less white water is from below Kiutaköngäs rapids to Jäkälämutka. Starting place for 25 km route is after Kiutaköngäs rapids near Mataraniemi fire site.
In May after river is freed of ice it is still quite wild but in June water level goes lower and stream is easy even for beginners. More experienced paddlers can try upper part of river from Savilampi to Oulanka Visitor Center before Kiutaköngäs rapids. There are also several rapids where river rafting trips go but Kiutaköngäs rapids is not one of them and not to be entered.
The easy route from Kiutaköngäs to Jäkälämutka can be paddled in a few hours but to enjoy the trip and nature more it is advised to stay a night either in Ansakämppä wilderness hut or on the opposite shore in Nurmisaarenniemi in a tent. There is also a nice fire site along sandy beach in Alaniemi near Jäkälämutka.
Paddling on the route you will probably see all kinds of wildlife; eagles, reindeer, tracks of bear perhaps? If you pass Jäkälämutka stop point by accident you will run to rope across the river indicating border zone. See more info of canoeing routes and rental services from nationalparks.fi.