I would say aerobics and strength training would be best. In the case of skiing, aerobics might serve you better, as well as balance exercises (yoga is good too :P). The main point is to keep your body in tip top shape until the on-season returns. Try to find another sport that suits your needs as well, or a sport similar to it or a sport that requires the same types of body movement. For example, if you play hockey in the winter, than rollerblading and field hockey/lacrosse might be good forms of keeping in practice.
