You can use any greens you have or love. Kale is so sturdy that it can be dressed an hour ahead and still not wilt or get mushy. Most other greens will not hold up as well, so you will want to dress the salad right before you serve it. Of course, the traditional lettuce used in Caesar salad is Romaine. However, I say try new greens! Even shaved Brussels Sprouts are delicious in this salad!