There is a recreational riding stable on Alpine Meadows Road that serves summer vacationers.  Rides include group trail rides in the forest, pony rides for children, and private rides of varying difficulty.  Their trail rides go through mature redwoods with views of the valley.

The stables are usually open June through September depending on the weather.

See their website here.

The pony rides for young children are especially appealing.  They have very tame ponies that a parent leads on a short trail through some forest and across a meadow.  It is much more fun than a ride in a corral.

Other Stables
Piping Rock Equestrian Center outside Truckee offers lessons and trail rides by appointment.