You can add some green grass. Cut it into short 1 inch pieces. Give them kitchen scrapes, bananas, apples, lettuce, melon. squash, bread... No egg shells at this point. Just limit what they get. If they get too much, you drop the protein content of their diet to the point where start feather picking to get what they need.
A little finely chopped green feed can be beneficial. No more than what they can clean up in about ten minutes or so. After a week or so you can cut it coarser once they become accustomed to eating it.
If you are feeding a fresh, quality chick starter they really don't need anything else. Adding more "stuff" to their feed can only negatively alter the nutrient composition.