Jump squats, jump lunges, walking lunges and Bulgarian squats. Put any sort of weight in one of your hands to challenge your stabilizer muscles during the exercise such as a weight plate, kettle bell, bowling ball, ski boot, wife's purse, ect.
As someone who's done a lot of conditioning, both ski and general, going back a lot of years, I stumbled onto something that's prepared me for skiing better than anything else I've done.
As part of a rehab program where I was unable to do weights or similar, I did a mile or two of walking lunges every day for about 6 weeks. First time ever it felt like mid-season at the start, body wise.
Before I was shut down, I had done a bunch of weighted exercises, core, ie squats, leg dips, etc, and was in good shape. Then a 3 month layoff with the walking lunges beginning about halfway through that.
This year I’m doing 4.5 miles of walking lunges about twice a week, Stairmaster 4 times/week, core four times/week and weights twice a week.