At the farm where I rode growing up, we used a play a trivia game where we passed around a wrapped package with a question under each layer of wrapping. They were mostly silly questions, related to the riders and our horses, etc. Everything was pretty tame but you could definitely spice it up for a group of adults, as long as everyone has a sense of humor! We had little prizes as well.
We always did a yankee swap gift exchange as well, probably not a “new” idea, but it was aways a good time! Much more fun than secret santa, especially with an all-adult crowd.