Last year I came into ski season fairly out of shape. I'm trying to avoid that fate this year. What cross training activities should I be doing that will pay off during ski season this winter? Trail running? Mountain biking? Yoga? I'm a new dad without a ton of time, so trying to find something I can do 2-4 hours/week.
You didn't say what type of skiing you do, so I will reply to this as if it could be any. For myself, I do all types of skiing (alpine, backcountry, XC, nordic skate). I find that a combination of cycling (both on and off road) and running during the off season is a great way to stay in shape for the ski season. It isn't a perfect science, however. For example, I still find that my quads need a bit of extra work when I start the season again, but a few weekends of skiing usually bring those up where they need to be. If time is a factor, you can easily take a few spin classes and run in your neighborhood.
If you backcountry ski, another idea is to hike with a heavier pack during the off season. If you are a nordic skater or XC skier, you can also get roller skis and use those around your neighborhood when you have time. It's a lot of fun and can be a good conversation starter with the neighbors 😉
In general, I find that if I keep my cardiovascular fitness up, it sets me up with the right foundation when the ski season starts. Trail running, mountain biking, road cycling, etc. are all great ideas (and a bit of yoga too to help stretch what will certainly get tight as you increase your activity level).
I admit it - when spring hits I'm a glutton. I tend to make up for all those months of working out for ski season and I usually pack the pounds back on pretty quickly. But come late summer I start watching my diet and exercising again. I don't have the time or space for a lot of equipment so my go-to is the elliptical machine at home. It gets my cardio going and after being back on it for a few months my legs are in prime shape. It's actually kind of nice to be exercising while being able to catch up on a few tv shows or movies.