My cockatiel got hold of potato chip, plain flavored, he relished eating it. However I did’nt know if it would make him sick. So I took it away. Now when ever I eat chips near the cage he gets all exicited. Is it safe to give him a potato chip every now and then in moderation? I have a parakeet in there with him because people don’t seem to keep him company. So what ever I feed him the parakeet might eat some too. Will the chips be ok for the parakeet? What about corn chips? While I’m at it. grapes? peas? pizza? Maybe just the crust? Bananas? Peanut brittle seems like a no no, but just incase. A sliver of peanut brittle? All things one bird or the other seem like they want some of if I have any within eye shot.
To be honest, none of that stuff is too good for them (except for the fruit), because of the fat and salt content, which is something their natural diet as primarily seed- and fruit-eaters is lacking, so their bodies aren’t used to it. However, I doubt an occasional treat is going to kill the birds on the spot. I wouldn’t let them have pizza, except maybe the crust with NO cheese on it. The cheese and heavy oils might cause gastric upset.
Well, ask yourself how often your 'keet would be eating trans fats or refined sugar or corn syrup in the Australian outback. Then allow him to have chips and peanut brittle as often as you reckon he would be finding them and eating them in the wild.
Junk food is just as bad for birds as it is for people. And I’m sure you can find people who will tell you, “Aw, go ahead and let the bird have his little treat”, the same way you can find people who will tell you, “Aw, go ahead and let the kid have McDonalds for supper every night…”
End of sermon. 
Give him grapes! It’s rather entertaining watching birds that size eat grapes, because they just kind of lick and nibble around the edge that was attached to the stem, getting the juice out of it. You could also try cutting the grape in half and putting the halves into a snack dish - not the seed tray; I always kept an extra food dish around just for snacks so any leftovers or dripping fruit juice wouldn’t foul the seeds and pellets. Peas and corn would be fine. Our birds used to like mashed potatos; we’d give them to them plain, scooping them out of our mixing bowl before we added the milk and butter, so they’d be healthier. Same with sweet potatos. Plain, unsalted, unbuttered popcorn can also be fun for the bird, since it’s light enough and of an odd enough shape that it should be possible for your feathered friend to get a grip on it and stand on one food nibbling at it adorably.
I wouldn’t be so sure that fats would be a problem. I had an African grey parrot that liked nothing more than palm nuts, and sunflower seeds, which seem to favored by many birds, have a high fat content. Salt, OTOH, I would worry about. I also wouldn’t feed a bird candy.
I’ve had several cockatoos that I offered a wide variety of food to. Each had its own preferences but cooked brocolli was fairly popular, carrots and peas wasn’t. IIRC, one of them liked cauliflower. They all took a little tropical fruit (banana, pineapple, star fruit, rambutan, etc.) but they seemed to have a preference for pears and apples. The only thing that all of them really, really liked to eat was chicken bones. The Moluccan’s would take a chicken thigh bone and eat the whole thing.
I’ve kept lovebrids, a conure, and a cockatiel. Yeah, they like junk food, too. One of my lovebirds figured out how to get into the sugar bowl one time and had a party.
If you’re going to feed birds table food keep it 99% the healthy stuff - unadorned vegees, fruit, and grains (my birds would all go nuts for rice). If once or twice a week (not per day!) you give them a nibble of potato chip, or a bit of pizza crust (actually, my lovebirds used to go for burritos) it’s not going to hurt. But make it a very occassional special-occassion treat. And just a bird-size bite or two, not an entire meal Assuming you eat anything like a healthy diet, the amount of oils or salt on things like vegees (without heavy sauces) shouldn’t be a problem.
I had one lovebird that used to eat cheese on a regular basis. I asked the vet about it, and she said as long as it wasn’t causing an upset (it wasn’t) it was OK. But then, lovebirds are much more omnivorous than either parakeets or cockatiels, with higher protein requirements - they’re actually little predators. My 'tiel and my conure never showed any interest in milk products.
The lovebirds and the conure also liked spicy food. Anything strongly flavored - my one lovebird would happily eat a whole clove of garlic. Ever sit next to a bird with bad breath? When he had his free-flight time he’d leave garlic vapor trails around the house.
The big problem with the fat content of junkfood and cage birds is that cage birds don’t exercise nearly as much as their wild cousins. Wild birds burn enormous amounts of energy. Cage birds, although they have a high metabolism, do not and it is possible for them to become overweight or even obese. They do need some fat and oil, but they also need exercise of some sort, too.
I have a blue-front amazon. He also goes nuts for potato chips and fries, hamburgers, sandwiches, or just about any other people food that has a heavy smell.
He’s always been like that so I think his old owner used to give him lots of that stuff.
I will give him a very small french fry or chip with all the salt wiped off from time to time. I so rarely have that stuff I figure just a tad wouldn’t be bad for him. I guess if depends on what you normally feed him.
That said, my bird always gets small amounts of rice, bits of bread, mashed potato, baked potato, and any veggie we happen to have with dinner for “treats”.
And, so far, he’s perfectly healthy.
I guess I should follow up with what I never feed him.
Chocolate, sugar, candy, avocados, meats, or anything with caffine.
The excess salt on foods like potato chips can kill birds.
Remember, they’re smaller than they look, under all those feathers, and what seems like a tiny amount of salt to us could be a lethal overdose to a tiny bird. Chocolate and caffeine are also potentially lethal to birds, so definitely avoid them!
Fresh fruits and veggies EXCEPT AVOCADO (which can kill birds) are good choices for healthy treats. Obviously, don’t feed the bird parts of fruit that you wouldn’t eat yourself. For example, DON’T give cherry pits or apple seeds to birds, because the seeds and pits contain poison.
Incidentally, seeds are also best used as a TREAT, not as a main diet. It’s usually healthier to try to get your bird to eat a pellet diet rather than a seed diet. With a seed diet, the birds often just pick out the tastiest seeds (like sunflower seeds) and don’t get a balanced diet. With a pellet, the nutrition and taste is uniform, so the bird gets a more balanced diet.
Read this article for some advice on cockatiel nutrition and how to convert a “seed addict” to a pellet diet: http://www.cockatiels.org/articles/nutrition/diet.html
With my parrot, I give a pellet diet (Kaytee Exact for parrots & conures - they make a formula for keets and cockatiels too).
For treats, I alternate between giving him Avi-Cakes (little squares of seed and pellet) and Nutriberries (round balls of seeds and pellets). I give him one of these birdie treats when he’s interested in my people food, and he usually is happy to nibble on that instead of begging for my food. 
OKay so if I understand correctly as a treat I can occansionally give them fruit with exception of avacodo, apple and pear seeds, unsalted potato products, rice, bread, veggies, and little bit of chicken bone. No sugar or salt anything. I should also switch to pellets.
My best friend’s budgie recently died because he had managed to snatch a bite of her guacamole - it took 24 hours. They aren’t kidding when they say NO AVOCADO!
Also, raw potatoes are aparently just as bad, though I imagine that´s less likely to be eaten by them.
Cockatiels (and budgies) are primarily seed eaters in the wild. A decent seed mix really can be okay for them. (My and my vet’s opinion. YMMV) Supplement with vegetables, fruit, sprouts, whole grain bread, etc.
The chip yours ate wasn’t particularly good for him, due to excess salt, but it’s easy enough to keep him away from them in the future. He isn’t a hawk, you know. Have other foods on hand to distract him from your crunchy, salted goodies. I have a tiel that will snatch pretzels, so I keep a small bag of unsalted ones for him.
The natural prey of free-flying budgies and grey-cheeks apparently is pizza cheese. We always had to cage the birds when having pizza delivered, or we’d always end up with an unwanted parakeet as topping.