Horses do not get arena sour because they are bored.
They get arena sour because they are drilled in the arena, then ridden out, petted and dismounted. When you think of it that way, where would you rather be?
Serious event trainers prevent arena sour horses that are being shown (where you cannot do what I suggested because the next performer is waiting to go into the area) by riding out of the arena, going back to the warm-up pen and riding the horse even harder than they did in their class. They make sure that the arena is not the place a horse would learn to dread.
I have watched reining trainers (myself included) ride a horse hard for 30 minutes right after coming out of a class just so the arena looks like a better place to be.
The worst thing you can do is ride out of the arena, dismount, unsaddle and put a horse up. You are teaching it to hate an arena and love leaving.