Pen enrichment for small herd owners

Since I have only 2 alpacas, a pair of geldings, I am interested in pen enrichment. Zoos require enriched environments for apes, chimps, and many other very intelligent animals. Alpacas in a larger herd probably provide a lot of entertainment for each other. But two alpacas alone probably could use some extra stimulation, especially in the winter when other external pen activities like passing animals, dancing cranes, wandering coyotes, farm machinery, and other human activity are not available. I am trying to learn more about the kinds of things would be good to have in a pen for my two animals to stimulate them but yet be safe. I spend at least 2 hours daily with them, but after that they are on their own. With longer days, they have many hours where nothing is going on, especially right now before the snow is gone. I would greatly appreciate ideas for enriching an alpaca pen environment. Thanks.

Nina