If you are going on a ski trip, you need to make sure that you do this exercise to prepare for the break but also for when you are out on the slopes.
When skiing you have to wear these chunky ski boots. The muscles that operate your foot and ankle do not get used very much during skiing due to the stability that is provided by the ski boots. This means that those muscles may waste and weaken, which increases the risk of injury to the area.
To help counter this muscle wasting, stand on one leg, and gradually increase the time you spend standing like this. Swap sides so both legs can get trained up. Ideally you want to start this a few weeks before your ski trip and continue whilst enjoying the vacation.
This will activate or reactivate those muscles to maintain their strength and function.