riding backwards? bareback? are they girls or boys?
i always remeber having a circle of poles and we had to canter around, picking a cup off the top and putting it back on the next free post. if you dropped it you had to gt off and pick it up. then you can do it in trot or canter. and then canter round in pairs, holding a whip with one hand, one person grabs a cup, gives it to the other and they then put it back down... i know its not really flatwork, but it does teach them to use their legs and control the pony.