This might sound obvious to experienced cyclists, but I've discovered that good sunglasses are a must, especially when riding through the countryside. Not only will sunglasses protect the eyes from harmful rays and the delicate skin under the eyes from wrinkles, but they also shield from the debris and insects that hurtle toward me at top speed.
Yes, insects! Assuming that your bicycle has no windshield, be prepared for your face to be hit by flies, bees, butterflies, and God knows what else. If you keep your mouth open, they will fly into your mouth. And if you don't wear sunglasses, they will fly right into your eyes or get trapped in your eyelashes. Based on personal experience, I can promise you that this is not pleasant.
To be useful as a shield, I find that the best sunglasses are those that cover as much of my face as possible. Conveniently, the "Jackie O." style is popular right now and every shop seems to carry them in many variations. I opt for cheaper sunglasses, because I have a knack for losing or destroying mine. The darlings pictured here lasted two weeks before I crushed them in my handbag -- which reminds me that I must buy another pair if I plan to cycle again over the weekend.