To get a first idea about night train routes, I suggest the following interactive map (it may be not up-to-date for some trains so compare the Information with the websites of the railway carriers): https://nachtzugkarte.de/

My first Idea would be heading up north and visit Göteborg or Oslo, then visit cities like Bergen or Trondheim by Night train. You could get all the way up to Bodo/Lofoten by night train.

Or you could also use one of the various night trains from Stockholm further north (Are/Lulea/Kiruna/Narvik). There are buses between Narvik and Fauske which connect the two railway lines allowing a big round trip mostly by rail. But as further you go up to the north, the longer the journeys get during the days.

Night trains can be reserved with the Pass fare on sj.se in Sweden and on entur.no for Norway.