At a sleep-away camp I once went to, the barn manager yelled at me for sitting down and saying I was bored. She said "You're in a fifty horse barn! You can find SOMETHING to do!"
My current barn only has 14 horses, but you can always find something. Refill and clean water buckets, cleaning tack, sweeping aisles... I LOVE doing carrot stretches in my spare time. Good for the horse and fun for them too! Sometimes if there is absolutely nothing (easy to do everything for 14 horses) I'll groom my horse really good, give her a bath, take her for a walk, or just let her graze.
"The wise man thinks he knows nothing.
The fool thinks he knows everything."