Do they do any WORK? (Going for walks, getting driven, roundpenning, etc.?) Horses need daily exercise. Best if they work hard enough to sweat and be tired. Give them something to engage their bodies and minds. It's boring to sit in a stall or out in a field and do nothing. That way, when they are not working, they are happy to just get some rest.
That said, my horses are worked daily but they are also stalled, so a little more prone to being "bored." They can see other horses and interact with them, which is a must for good socialization. They can keep each other entertained. My horses have very busy minds and like to play, so I have plenty of toys for them. They all have a selection of jolly balls, ropes to chew on, Pony Pops to lick, traffic cones to drag around and destroy. I think my colt would go nuts without his toys.
Andrea