I top dress the chick mash with a few dried mealworms, which you can usually buy from the feed store - just be crumble them up until they're a couple to three weeks old! Also, there's a great product from MyPetChicken.com called Chickie Puffs... they also have Chicken Caviar!!
I started giving my chicks treats within the first week but I made sure they had grit available several days before they got any treats. I have given them worms, grubs (with heads cut off), chopped up strawberries and dried mealworms. Some of them have no interest in any of the treats. Be sure to check out the treat chart
Mine started eating broccoli when they were about 10 days old. Chop it up really small at first. Mine absolute loved it and it is still their favorite food. At about 2 weeks they started eating strawberry tops (cut up), cooked rice, small amounts of cheese (it has a lot of salt in it) Shortly thereafter they ate veggies -corn and peas are their favorite - and hard boiled eggs.