There is beautiful lake in the park where you can swim if you don't mind the cold water. There are not many spots where it is easy to get to the water so if you get to some good spot use it as you don't know how it will be in the next stop.
Oulanka is probably one of the most popular national parks in Finland. In the summer it might be a bit crowded but by no means, it does not get that kind of crowded like a popular place, it just that you might not be all alone in the forest... :)
Oulanka National Park
I am not actually sure if there is any place in Finland that really called Oulanka, the name is of the area where is also this fantastic national park and if you are ever plan to get to the north east side of Finland you must reserve couple of days to explore this national park, by no means one of my favourite in Finland.
Oulanka National Park is also popular for bird watching, there are few trails and spots that recommended for this activities and if you want to know more check the link included here of pro sites.
The River Oulankajoki
The River Oulankajoki is what probably gives the name to this park, it is going through the park with few nice wild rapids in the upper part and more slow and calm in the lower part.