Crossfit is definitely even worse idea with the compound movements etc if you ain't aware of the right techniques. Especially when you get tired there's a big chance to get injuried, so i wouldn't recommend it for nothing else than own bodyweight excercises when you're somewhat beginner or not aiming to be a competitive crossfitter. Those push ups, sit ups, pull ups, squats, box jumps etc should be completely fine in the beginning to boost up your vo2 max and have stronger muscles. The best thing with crossfit is that it doesn't require much time and you get something for your lungs and muscles. Another thing i'v found very useful indoor season to keep myself fit is indoor rowing. I don't really like to ski or anything, running is out of the question in the winter so rowing works perfectly there. 30sec intervals, 30sec fast and then 30sec slowly and you can pretty much do 10 sets, 1 set being 30+30seconds so it takes about 10 minutes after the gym to do and there's a guaranteed sweat around and your lungs are pretty much dead.