Catnip (Nepeta Cataria) and/or Cat Mint (Nepeta mussinii) are, of course, a must. Each cat has her own preference. Some like catnip better, and others catnip. Some cats, like mine, are even "catnip connoisseurs", preferring the taste and scent of one individual catnip plant over another of the same species.
Cat grass is essential for indoor kitties. This can be grown from seed, or purchased in small containers in pet supply stores, already sprouted and ready to grow. I found a small windowbox container in which I have sprouted a small "lawn" which my little orange boy keeps very evenly mowed. This alone may be enough for some cats. If your cat goes outdoors, he may be happy to mow your back lawn for you, to the best of his ability.
Dr. Leonard Perry of the University of Vermont also recommends Cat Thyme (Teucrium marum)and Valerian (Valeriana officianalis), though he warns that rodents also enjoy Valerian. This may be fabulously entertaining to your kitty, but not so attractive to humans, especially if your feline herb garden is indoors! He also suggests some interesting features to add to an outdoor cat garden to protect plants from overly-enthusiastic kitties, and to encourage play and frolicking.
I have also read that some cats enjoy parsley and sage.
Some cats also express interest in vegetables eaten by humans. Mine loves canned pumpkin (plain, not the sweetened pumpkin pie filling), which is a good treat for kitties according to Wellvet.com. Some cats also enjoy raw pumpkin:
My furry companion will also ask for lettuce on occasion which, according to the ASPCA, is safe to serve your cat. An old college friend of mine has a cat that craves asparagus. I have no idea if this is safe or not. Spinach is a bad idea though. I have read it can give kitties kidney stones. That can't be pretty.
Indoor plants not only provide nutrition and entertainment for your cat, they can also keep him from eating your ornamental specimens.
Speaking of ornamentals, it is important to note that some cats still do not confine themselves to plants intended for their use. The really determined ones may even attempt to nibble synthetic plants! If this is the case with yours, or if you are unsure, please keep plants not specifically recommended as feline edibles away from your cat.
As always, when in doubt, ask your vet!