As a fair-weather cyclist I've found that my time spent on the bike has made a huge difference in my return-to-skiing fitness. I have a love of the road bike, but the mountain bike is even better at simulating a ski experience for the whole body. Rapidly changing surfaces full of obstacles, the importance of knowing how to fall, and the need to remain relaxed in order to absorb terrain changes with the knees are all great practice for skiing. But the most important contribution a bike can make to your skiing is the fitness gained by your thighs. In this case both the road bike and the mountain bike will contribute greatly to a smooth transition to snow when the winter returns.
Another way to train in the summer for skiing without knowing it is to slip on a pair of inline skates and get rolling. The balance, leg angulation, and body position of skating are great crossover skills with skiing, and once again the work the thighs are asked to do translates directly to the skis. And skating, like mountain biking, is very good for practicing other important skills needed in skiing, such as one of this month's skills focus: watching the space between the trees rather than the trees. Whenever an obstacle is encountered, keep your eyes on your route and your body will follow.
Hiking and backpacking offer an important ski fitness component that is essential to maintaining leg fitness - downhill walking. When you are hiking uphill you are working on all sorts of fitness elements that will serve you well when the snow flies again, but the downhill portion is a much closer simulation. The quads, ankles, back muscles, and knees are all being asked to remain both relaxed and controlled, a debt your legs owe skiing that it will not forgive. The added weight of a backpack is another great way that hiking helps the legs.
Trail running also works the legs both up and downhill and is similar to mountain biking in that you cruise across varied terrain at fairly high speed. Like hiking, trail running demands quick reaction time, a quiet upper body, and quads of steel. All things that will ease your return to skiing.
The time you spend playing at other sports has another less obvious contribution to your fitness. After a winter of snowboarding, downhill skiing, snowshoeing, or cross-country skiing, your body needs a chance to repair any damage you may have inflicted upon it. To help the body in its repair efforts it's important to stay away from activities that aggravate an injury. If the pounding and added weight of hiking bothers your knees, the controlled, rhythmic motion of cycling may be acceptable.
Participating in one of these sports as few as two times a week through the summer and fall will have you ready for winter when it starts, rather than wasting precious powder days getting strong.