When I got Mealworm Frenzy for my chickens one time, they went CRAZY for the mealworms. They'd gobble 'em out of my hand faster than chickens on a slice of bread. As for treats I give my chickens more often:
Yogurt
Cheese
Scrambled Eggs
BREAD!
Black Oil Sunflower seed
Suet
Leftover meat scraps...