Have a medieval fair. Invite the kids to dress as princes and princesses. Let them decorate their ponies with fingerpaint. They can make circlets from pipe cleaners and plastic flowers. They can pretend to "joust" by riding at objects like an apple on a fence post and knock it off with a stick. They can make up a skit on horseback with a medieval theme--rescuing a princess from a dragon, none shall pass, crossing a brook where both riders insist on crossing first, act out Sir Gawaine and the Green Knight-- many different scenarios. They can go Maying by riding out, picking flowers, and making flower chains.
Have a cowboy theme. Again, cowboy costumes, paint the ponies like Indian ponies, practice roping a post, knock the hat off the post. Tie a barrel to pieces of inner tube and ride the bucking barrel.
Have a Native American theme. Costumes, of course. Practice riding
bareback, make Native American crafts, cook lunch over a fire. Have a "thread reins" contest using yarn instead of reins to see who can continue more and more difficult paces without breaking the yarn. Throw a "lance" at a buffalo.
Star Wars or on-line gaming theme with similar games and projects.
My Little Pony with similar games, pony painting, and crafts.