Need a new plaything for your curious kitten? The Kitt'n Gym is just the ticket--for kittens of all ages. Made of sturdy but lightweight and washable polyethylene plastic, this toy is hollow and ball-shaped, with several cat-sized holes and a catnip-filled sock suspended inside. It's available in a variety of colors and in two sizes: one is 13 inches in diameter, for kittens, and the other is 16 inches, for larger cats. Order by mail from Cal-Pet Products, P.O. Box 6119, Fullerton 92634; or call (714) 992-4600. The smaller size costs $29.95 plus $3 shipping and handling; the larger sells for $39.95 plus $4. Add 6% state sales tax.