Specific exercises for skiing and snowboarding endurance?
I don’t ski all that often, but I need to prepare. What are some exercises that I can do to get in shape so that I have strong endurance all weekend on the mountain?

While skiing, you will mostly rely on your leg-muscles, more precisely your ankles and knees. So, you will have to make sure you train them a lot.
Next to that, you will need to have stamina. Swimming is good for building up stamina quickly.
Start training about 2-3 months before you go on holiday. This will give you enough time to build up some extra strength, without having to keep it up for a prolonged period.
Personally, I do a lot of exercises that I need to bend my knees for, and keep that for a longer period. One very good exercise is grabbing a vertical pole (like a door-post), stand on one foot, and ’sit’ down until your leg is at a straight 90 degree hook. Keep that up for about 30 seconds, and switch legs. Repeat this as much as you want/can.
Another good one is stand on one foot, and then try to stand on your toes with that one foot. Repeat 15 times, switch feet, and also 15 times. Repeat as much as you want/can.
A third good one is to make ’skating-hops’. Stand on one foot, bend over like when you are speed-skating, and hop to the other leg. Hop back, and repeat as you want.
The link below provides even more insight in the type of training you can do.