You may substitute almost any vegetables as long as you keep the base of raw cabbage and salt. Some good substitutions include bok choy, celery, leek, fennel, collard greens, cauliflower, kohlrabi, and rutabaga.
Ruby Rye Kraut:
Substitute red cabbage for green cabbage. Omit the apples and juices, and add 2 to 3 tablespoons of caraway seeds.