I've said this before and I'll say it again: get your horse out of the perfectly manicured arenas.
It will contribute to their cross training, their soundness, their confidence, and so much more.
Be smart about it, know your horse and take baby steps if you have to. But get your horse out and about.
I took Charlie for his first solo trail ride the other day. We started with some solo rides but within eyesight of other horses when we were down in Aiken. Then we went on the trails back home here in Ohio with a steady eddie the other week, then he finally went on his own the other day.
Not only is it good for his body and balance to work on different footing, and up/down terrain (he's not very good at walking downhill yet!), but it is also good for his brain: 1) to not always be stuck in an arena And 2) to know that he can go out on his own, with no other horses, and be perfectly fine. That builds confidence in himself.
A confident horse is a safer horse. And a confident horse is more capable of higher performance.
Not only is this applicable to all disciplines, but obviously as an eventer, it is vital that our horses are okay with going out on their own. They need to be able to leave the herd, whether it's to do a dressage test, a jumping round, or the most "alone" phase: going out on cross country.
It is so incredibly vital that your horse has absolutely 0 hesitation and 0 worry about where other horses are. If your horse is even minimally distracted or influenced by where the other horses or barns are, that will be one more thing that you will have to overcome out on course. So eliminate that issue and get your horse comfortable and confident out on their own.
Michael Willhelm Equine Academy