I like to feed watermelon in the summer time. If it's frozen- it turns into a horsey Popsicle! Keeps them busy, and the natural juices can help keep them hydrated. (Plus watermelon is pretty cheap in the summer, so that's always a bonus )
I recently made a friend in a horse that loved strawberries. I had been eating them and fed him the tops of the strawberries that I didn't eat.
Granted, I don't know that he actually tastes what he eats, he just inhales whatever you offer him, lol. A lot of horses I know enjoy bananas (even the peels!). Most fruits are okay to feed to horses as far as I know. There are some veggies that they shouldn't eat, so just do your research beforehand (as I can't recall any off the top of my head, but I know there are some).