Carrot peels- contain the highest concentration of nutrients. Scrub carrots and eat whole or sliced to take advantage of all the carotene and other antioxidants.
Potato skins- are loaded with fiber, vitamin K, potassium, copper and iron.
Apple skins- Are rich in vitamins A and C. Nearly half of an apple's vitamin C content lies beneath its skin.
Orange peels- contains more than four times as much fiber as the fruit inside, and more tangeretin and nobiletin—flavonoids with anticancer, antidiabetic, and anti-inflammatory properties. Use that zest!
Banana peels- This is the one I haven't yet tried. Anyone tried this one? Apparently, they have a lot of fiber and potassium as well. I guess you can cook them. If you've tried it, let me know!
Onion skins- Simmer in a stock to get more flavor, as well as antioxidants.
Cucumber skins- have high amounts of dietary fiber.
Eggplant skins- are packed with the phytonutrient nasunin.
Kiwi skins- when eaten, can triple the fiber intake! They also have a lot of vitamin C. They take a little getting used to, but I really like the extra zip!
Mango skins- have good fiber. Some people are alergic to them. i usually eat some, but not all of the peel. There are more studies being done on the health benefits, see here and here.
Any fruits or vegetables you don't peel that I missed, or ways you recommend preparing them?