You could have the kids run games that are played at horse shows.
Pole Bending Race: Running between poles that are close together with out touching or knocking them down and timed.
Barrel Race: Running to three barrels set up in the yard.
Egg and Spoon Race: Everyone races across the yard with a raw egg in a spoon. Could let them hold the spoon in their mouth while they run if you want it harder.
You could get some ropes and have a lasso contest.
For a craft you could call a farrier, they usually are more than willing to let you have some old horseshoes the kids could paint. A large animal vet could be another source. Or just stop at a place that has horses. People with horses usually have a old horse shoes laying around.
Of course playing a game or two of horseshoes would be fun.