Depends on what you’re thanking them for, I guess.
I think a genuine handwritten card is often enough. Most of us don’t do favors for the little prize we might get in return. I also tend not to do thank you’s for people I pay-- like the BO or my trainer. I might give them a plate of cookies/something at Christmas but not much else. Maybe I’m a cheapo.
Otherwise-- we have a chocolatier relatively close to me that often makes it into thank you’s and small Christmas gifts. Gift cards to coffee shops or even the grocery. A bottle of wine or a pack of local or craft beer-- my friend got a new cat and I gave her a four pack of Laser Cat. I like to give gifts I know will be used or eaten, but I also don’t like “stuff.” There are shops that do personalization for not much extra money. I’ve given coffee mugs, water bottles, and wine glasses with logos or pictures on them and people have really seemed to enjoy them.
For horse people: if it’s between October and April, hand warmers or toe warmers. Otherwise, high quality horse treats, I’ve given people patterned or bright polo wraps, or a gift card to a tack shop with just a little money on it. $25 is still nice when you spend $200 on some horsey item.