You post that you give them little jobs but you do not say what those jobs are.
Alright I'll explain myself some more.
If the horse loses focus on me I take them in a nice circle, or if space doesn't allow that, I weave the path instead of taking it straight, I leg yield from one side to another, if I could sidepass (haven't learned that yet) then I would add that in too. I add transitions from walk to trot and back to walk. I check to see if my horse is paying attention by halting them and making them stand, backing them up, encouraging them to flex from side to side. I add figure eights into large areas, working on keeping him bent.
Now with Sky, when he does good, I give him a nice loose rein and we go forward. If I feel his focus is dwindling, I do one or a few of the above.
When we encounter something he doesn't like on the trail, like if he's spooking at a dog, I bend him away from the dog and give him other things to focus on (as mentioned above) than the dog. If I let his mind think about the dog, he will bolt off because he's nervous around them.
But usually he's very good about listening to me and we don't do many "focus on me" exercises but I do make him halt and wait to continue. I do leg yield him at times to see if he's paying attention, and when we encounter a dog, I help him get past it because I am his herd leader, therefore he knows I won't lead him into danger because in the past I have kept him safe and kept him out of trouble.
It all depends on the horse and the situation but I hope that helped this time. I didn't realize I was being vague.