WHEN are you going to do this? Because I was born and still live along your route plus did the same route this June (the other way round) by car so I guess I might have something to suggest, but the season has a real big impact on what to see...
-on the Swedish coast, I would stay overnight at "Höga Kusten" or "High Coast" - - a scenic part of the shoreline
-the twin town Haparanda - Tornio is real small but the river is very beautiful, and if I were you, I would go a bit upstream, eg. Kukkolankoski ( http://www.haparandatornio.com/see-and-experience/Kukkola-rapids/232 ) (In historical times, scientific expeditions and travelers used to go that way up to the North, for example Giuseppe Acerbi, French academics etc..)
- Kemi with it´s Snowcastle is a must, if you go in wintertime (the city itself is not worth a visit) ( http://www.visitkemi.fi/en/snowcastle )
- If you go in summer, the SnowCastle won¨t be there...:) but there are some nice exotic accommodation possibilities ( http://glampingfinland.fi/destinations.html
for more on these things )
-In Finland, Central Finland is very beautiful (the coast has some nice spots and wooden old towns like Rauma or Raahe or Kokkola, but nature isn´t that nice...