Behavioral enrichment is a concept pretty widely known in zoo animal husbandry but not as commonly known / used in the equine industry.
Horses are, by nature, social animals. They crave interaction and socialization. Of course, not everybody can afford two horses and multi-horse paddocks aren’t always available or feasible.
Horses are meant to graze for a big part of their day. Something as simple and easy as providing a slow feed hay net for them to munch on throughout the day can help to keep them occupied for larger parts of the day.
There are a wide variety of stall / paddock toys available commercially but you can also DIY your own on a budget!
Horse’s can get “bored” with their toys too so it is best to mix it up. Maybe this week it’s a Jolly Ball, Next week it’s a treat roller, and the following it is a traffic cone. To keep things interesting for your horse, keep it new by rotating toys.