Hay and/or pasture should be available as much as possible. Leaving a round bale out for them is fine, though you may want to put it in a net or ring made for horses (don't use cattle rings) to minimize waste.
For hard feed, it's best to feed as consistently as possible. Horses have rather delicate digestive systems and changing feeds or feeding irregularly can cause colic, diarrhea, etc. Cereal grains (corn, oats, barley, etc.) are not well digested, don't offer a lot of nutrients, and are higher energy than most horses need.
A ration balancer, like Endiku mentioned, is kind of like a vitamin/mineral supplement with some added protein. It's low calorie, (usually) low sugar/starch, and designed to fill in the holes that are left from a pasture/hay diet. They're usually fed in small amounts (1-2 lb per day for an average 1000 lb horse in light to moderate work). Because of the small portion size, you can get away with feeding these only once daily, but it's even better if you can split it into 2 feedings.
Hay/pasture with a ration balancer is a great starting point for most horses' diets. If the horse doesn't maintain weight on that alone there are many easily digestible, non-grain alternatives to provide additional calories. My usual go-to's are alfalfa (hay, pellets, or cubes are fine), rice bran, flaxseed, and beet pulp.