look up some ab, leg, and back stretches, then idk get creative
try thinking of all the stretches you did in sports over the years (stand on one leg and pull other back, try to touch your toes, pull your knee to your chest, stretching your calf by pressing your hands up against a wall, etc.)
Obviously, the best one for on the mountain IMO is when you're on the lift and you grab the nose of your ski, hold it, then eventually start grabbing lower and lower down. Best calf stretch in the world before skiing.