What would be a good thing to do with your horse if they don't out when it's raining?

Ground work. Or play with your horse in their stall and do stretches. Teach them new tricks.
Lots of ground work!! use trot poles and lunge him over them or set up some free jumping/ mini jumps and jump him over them. You could also use some time just to bond, so groom your horse and just give him some TLC
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