Many horses like watermelon! You can cut that up into cubes and freeze them before you take them to the barn so when you feed them they aren’t too warm. You can feed your horse a banana too. They can eat the peel so you won’t even have to peel it!
I’ve heard you can feed them celery too... never tried it though.
There are a few other fruits that you can try out too but these are most common other than apple and carrot.