I wouldn't give them electrolytes unless there's something wrong with them and you're using it as part of a treatment plan. Their feed has vitamins already added to it, so I'd be careful not to overdose them on vitamins that aren't water soluble, like vitamin A.
Letting them eat some grass and fresh leafy greens is very good for them. That's a good way to give them more nutrients in a way that's safe. And those fresh foods have even more good things in them for the chickens than the packages of vitamins do. I plant extra chard, spinach, kale, leaf lettuce, beets and a long list of other veggies, just for the chickens. They are really funny, when you lob a cherry tomato to them. Bugs are really good for them, too. They're a healthy source of quality protein. A small amount of sunflower seeds is good, too.
A lot of people use vinegar. I don't normally give it to my chickens, as I haven't had any need to. I do use a lot of it in the house for cleaning, though! I'm sure a lot of other people will be on here as the day unfolds, to give you more opinions.
