Horses are programmed to eat small amounts of food on a continuous basis, so your horse will ALWAYS want another treat, but for his well-being, learn to say no.
Almost any fruits, and many vegetables, are safe treats for healthy horses. Apples and carrots are traditional favorites. You can safely offer your horse raisins, grapes, bananas, strawberries, cantaloupe or other melons, celery, pumpkin, and snow peas. Most horses will chew these treats before swallowing, but horses that gulp large pieces of a fruit or vegetable have a risk of choking. Remember to cut treats into smaller pieces before feeding. A few sugar cubes or peppermint candies (one or two) are okay, as are many of the commercially available horse treats sold in equine catalogs.
Treats can be fed by hand or by putting them in a bucket or feed trough. Some horses that are hand-fed tend to become nippy; others have better manners. Using a bucket is probably safest, but if you want to feed by hand, put the treat in the middle of your flat hand and think about pushing it slightly toward the horse’s mouth rather than withdrawing your hand as he reaches toward it. This inadvertent action by the owner is often what causes the horse to lunge for the treat. Don’t get in the habit of feeding treats every day, and certainly don’t give your horse a treat on a regular basis such as after each lesson in the ring.