Cats are carnivores; they primarily eat small rodents, birds, and bugs. Because hunting is such a big part of a cat's life, even indoor cats want to engage in hunting types of activity.
Keep this in mind when shopping for toys for your cat; most prefer those that resemble mice, birds, and bugs (identify your cat's "prey preference"). If you really want to make it realistic, make the toy move like it really is alive! (See more information about toys).