Where I live I can get raw peanuts dirt cheap. Would that be a good little protein boost? Can chickens even eat peanuts?
Yes, peanuts should be fine. They are a fairly good source of protein, and I think they have the right amino acids to pair well with grains (
@U_Stormcrow am I right that peanuts, being a legume, have amino acids that help balance the ones usually present in grains?)
And as
@Perris pointed out, any kind of meat/fish/milk/cheese are good protein sources too (yes including dried crickets, but dried bugs are usually overpriced in the USA, and might be in your area. If actual meat or fish is cheaper, it is certainly fine to use.)
Is cooked rice and couscous OK too as I always have leftovers.
Yes, also fine.
Chickens can eat almost everything that people eat. If you offer them everything, and let them pick through to eat what they want, they almost always make safe choices. So I wouldn't stress too much about which things are "safe" for chickens to eat. Nothing is going to kill them with just a bite or two, and they are usually smart enough to make the right decision by that point. Chickens that are used to having food scraps and foraging for part of their own feed are usually very good at this (because they practice all day long!)
Also I've added a picture that's pretty close to what the breed of chickens here are (it's just a photo from the web about breeders in south Africa) they look pretty similar, except for physical condition. They seem to be laying pretty well still though.
The ones in that photo do look like layer-types, so hopefully the ones in question are suitable too.
I will definitely get chicken feed, even if I'm sneaky about it
If it's OK with the owner, there is no real need to be sneaky
And I would definitely check prices-- you mentioned peanuts being cheap, which is not something I would have thought of (because they aren't really cheap where I live.) Chicken food is cheap where I live, which is why it is my default advice, but it might or might not be cheap where you are. There more expensive the chicken feed is, the more it is worth looking carefully at what else is available.