In my many years as a kindergarten teacher in a Waldorf school, playing horse games and horse riding games was a favorite playground activity of 3- to 7-year-olds! The reins were usually the first thing children would ask for when it was time to go outside.
One child becomes the horse, donning a pair of reins, while another child becomes the rider holding the reins, trotting happily indoors or galloping energetically outdoors.
We used these same German-made leather reins at our school, which held up year-after-year through much active play.