I am assuming they have a lot of equipment and don't really need anything. Is there anything that needs to be painted or repaired? (Like there never is at a stable, lol) Possibly put in a couple hours painting fences, walls, etc.
Another good idea is homemade horse treats- do a search online and you can get a ton of recipes. Another thing I saw was holiday bran mash for the horses, including supplements like msm and glucos.
One thing that never fails is a certificate with some higher end instructor in that field (jumping, hunseat, dressage?). Everyone takes lessons, and they can get costly after a while. This is quite a hefty price sometimes depending on the level of riding that your instructors do.
Or: customized name plate for the barn or the horses. I am talking a wood or metal sign with the name on it and some horse in their discipline.
One more: a customized embroidered jacket/shirt with their logo on it, or their name and a figure (hunt, jump, or dressage)
You never specified a price range, and these vary quite a bit.