Most non competitive horses in light pleasure work will cope just fine on good hay alone. Mine don't do much in the winter and I'm loosing them down now as once it snows its just 'ticking over' only, a couple of days a week at a local indoor arena or on our own if we can keep the snow and ice off it.
I give them just a small amount of Sentinal senior with a bit of sugar beet and either chop or hay stretcher pellets - so little its more of a treat than a feed.
I add a magnesium and chromium supplement too it
As for high energy - that might be just how they are, mine are like that if I fed them on cardboard but I have a friend who could feed her (healthy) pony on rocket fuel and it would still plod along