The most accessible area by train above the Artic Circle is Abisko on the Stockholm - Boden - Narvik line.
However honestly travel isn't very easy, especially as most night trains are already sold out over the Christmas & New Year's period. I haven't looked at accommodation!
The usual route would be:
- daytime Rotterdam - ... - Osnabrück - Hamburg, time for dinner
- night train to Stockholm
- 1-2 days there
- night train to Abisko/Narvik
Roughly the same thing back.
I found a few things with availability such as Stockholm - Boden C leaving on 30th December evening. Then early in the morning you switch to the seating carriages and continue to Abisko or Narvik. Arrival on 31th December around midday.
So you'd leave Rotterdam on 27th December, travel to say Copenhagen via Hamburg. Spend a night there and continue to Stockholm via Malmö. 29th full day in Stockholm.
For the return...
Note the mandatory reservations on Swedish fast trains, also recommended on Hamburg-Copenhagen ECs. Night trains cost extra too, price depends on couchette/sleeper, private or shared.
Feel free to ask anything. Btw a 25% Interrail sale just started. :)
Mmmm I've found a way better option! The Duved/Åre area is still pretty far north and snowy. :)
For example on 27th December:
- ICD Rotterdam C - Amersfoort C 07:11 - 08:20
- IC Amersfoort - Osnabrück Hbf 08:36 - 10:51
- ICE Osnabrück Hbf - Hamburg Hbf 11:23 - 13:14
- margin for German delays, time for lunch
- EC Hamburg Hbf - Copenhagen 14:56 - 19:38
- night in Copenhagen or Malmö
On 28th December:
- frequent Copenhagen - Malmö train
- X2 Malmö C - Stockholm C every hour
On 30th December:
- night train Stockholm C - Åre or Duved 22:39 - 08:15 (book ASAP)
And the return… night train Stockholm C - Hamburg if available. Otherwise similar route as the way in.
I haven't looked at accommodation or activities, that's your job but I'm sure you can find some things to do!