I find that people are so quick to give gift certificates. I have really gotten to know the staff at my barn and I think it is a lot more personal to get someone something they really like, need or want.
If you want it to be a horsey gift why not do something cute like a board game geared towards horses. I just picked up a Horse Monopoly game. Board games are awesome fun and a great gift to get other people involved and smiling.
I was a barn manager, and the boarders gave me tips thruout the year and on X-mas I would get some cash and some more personal gifts like pictures of me and their horses which I really loved. When I left to move to another state they threw me a party and I got horsey jewelery and more special pics and some traveling money. Needles to say I had really good boarders that really respected and loved me. I think the pics of me and my d(their) horses were the most special of all.