To add another one about pictures: One year a fellow boarder bought a Santa hat (with an elastic band that went under the horse's jaw to help hold it in place) and one of those long streamers of tinsel decoration rope (stuff, thingamabob, I hope you know what I mean). She put the Santa hat on the horse and wrapped the tinsel rope around their neck/shoulders and took pics of each. This was back in the day before digital cameras, so she had the film developed, put in cheap colorful plastic frames and gifted each boarder with a cute Christmas pic of their horse.
(now, I know some people might not appreciate their horses being "accosted" this way, but everyone at the barn was pretty laid back and no one minded this happened without their knowledge.)
I like the brass plates that go on the back of the saddle or the round tag that goes on the bridle. Back when there were tack cleaners at the barn, these were used to make sure you got your own tack back.
They're definitely old school but I've found most people really like them (and they're personal).
I think a small "generic" horsey gift basket would go over well. Put in a small bag of horse cookies, some candy canes, a cheap leadrope, a hoof pick, or a brush, and maybe some human treats! Throw them in one of those little 2.5 gallon buckets or a cheap grooming tote type thing and it makes a great gift!
I don't have a problem with treats. Several years ago one of my fellow boarders got some cute Christmas-decorated plastic storage jars with a flip top. I still use it to keep treats in the barn as well as the tack locker key for the girl who half-leases the horse.