The treat should be relatively small and quick to eat. I won't base Scooter's diet on things that aren't too healthy, but the reward MUST be something that the animal REALLY wants or it loses value. I use treats that I don't normally include in hit diet.
My list sounds awful, but keep in mind that it is only a tiny portion of his daily intake. My vet knows about it and approves, she told me that Scooter is one of the friendliest and best behaved Cockatoos she see's. In fact, I'm still trying to get Scooter to forage (rather than dismantle all the screws in the toy and toss the treat inside at me in disgust) and she told me to use something he just can't refuse, like a gummi bear, a frosted cookie, or a cool ranch Dorito. (She also tells me that dismantling the toy IS foraging, it's just doing it the hard way, he's probably bored.) I was trying to use healthier treats like nuts and dried fruit.

Keep portion size in mind - Scooter is almost a kg. Use smaller pieces for smaller birds. Bird Brain has a sweet tooth:
almonds - in shell (he LIKES taking them apart)
mini-jelly beans
mini-gummi bears
a portion of a starburst
small slice of hot dog
corner of a cool ranch Dorito
small chunk of steak
small piece of graham cracker
small piece of a cookie
corner of a frosted fruit pop tart
large sunflower seeds - in shell

Once again, these items are NOT a regular part of his diet, but rewards when he's being particularly good.
We've had a particularly challenging week, and his weight is down, so I've been giving him a mini-lollipop when he let's me examine him and administer his antibiotic. I have to talk to him and slowly start preening his head after the ordeal, if he's too wound up he'll toss anything I give him back at me in disgust.

Nothing is so strong as gentleness,
Nothing so gentle as real strength.

Saint Francis de Sales