Chickens love to eat. They are like little pigs and the quickest way to get them to come running to you is to go out to them shaking your bag or rattling a box or whatever you use to give them regular treats. They will eat pretty much anything, but some favourites seem to be raisins (you may even be able to get them to jump for those!) and mealworms. Mine will literally do anything for mealworms and if you ever have trouble trying to get your girls (and boys) to go into their coop or pen my recommendation would be to try throwing in a few mealworms and see if that helps
Treats should ideally not make up any more than 10% of what they are eating throughout the day as their layers feed is what they really need. If you notice your egg numbers dropping, try reducing the treats you are giving and see if that helps. As with humans, healthy treats are obviously better for them. Chickens love corn but there is very little nutritional value in it so that would be one thing that could be given as an occasional treat (as opposed to a staple of their diet).
A nice treat on a hot day would be something like water melon (chilled in the fridge will give it extra coolness). I once gave mine some strawberries that were a little past their best and the chickens loved them but I got a bit of a fright when I saw gooey red blobs around the run and thought they were blood until I remembered the strawberries they had earlier in the day.
Incidentally, if they eat blueberries their poo will turn purple and beetroot will make it pink!
Enjoy finding out what your chickens like to eat, but avoid things that are salty, sugary and anything moldy. Here are a few specifc foods to avoid.