Lettuce, Bok Choy and cabbage are relatively easy to grow from scraps. Instead of throwing out those leftover leaves, simply place them in a bowl with just a bit of water in the bottom. Keep the bowl somewhere that gets good sunlight and mist the leaves with water a couple of times each week. After 3 or 4 days, you will notice roots beginning to appear along with new leaves. When this happens you can transplant your lettuce or cabbage in soil.
Celery is one of the easiest foods to grow from leftover scraps. Just cut off the bottom or base of your celery and lay it in a bowl with just a bit of warm water in the bottom. Keep the bowl in direct sunlight as long as possible each day and after about a week, you will begin to see the leaves thickening and growing along the base. When this happens, you can transplant your celery in soil and wait for it to grow to full length.