The reward is not what WE say the animal should want. Both kicking up their heels, and standing still belong on the list, as does a vigorous massage, time with a favourite pasture mate, or a chance to roll in a sand pit. The problem with this list is obvious. It's hard to use these things in a training session. You can't let your horse drop and roll every time he gives you a right answer.
Timing is another factor in choosing a suitable reward.
Without a bridging signal rewards need to be delivered exactly when the behaviour occurs. That way the horse can clearly mark what it was doing and repeat it again for another reward. Delays between behaviour and reward can lead to confusion. You think you're rewarding your horse for dropping his head. He thinks it's for swishing a fly with his tail.