For when just skiing is not enough for you.
I was quite happy to discover that my legs are in good condition after Saturday's ski day with no soreness Sunday or Monday. My triceps and abs were a bit sore however. This is from doing one of my very favorite "workout" routines for cross country skiing, called double poling, or more accurately "short arm double poling".
I was able to find a clip of a guy with good form to show you how it's done.
You will notice that most of the work of moving you forward is done with the contraction of the core, the arms adding to forward momentum only at the end.
When you get the breathing coordinated with the movement you have a very powerful, explosive movement that works your whole body. This can be carries over into any number of other sports.