As others have mentioned, “Da Brim” makes a great product! I used one in the last RAGBRAI, and would recommend it 100%.
I opted for the full size version called The Classic, and found that wind was only an issue in a few brief instances (fast downhill into headwind), but not an issue 95% of the time. The added brim works well with most helmets, but I suggest making sure that you have a well fitted helmet that is comfortable.
I bought mine from the DaBrim.com website. They are also sold by some vendors the day before the ride starts, but I would order now so you can get the color, style, etc. that you want.
In addition to sunglasses or a helmet visor such as the one made by Polar Optics, I strongly recommend a light colored or white, loose fitting (but not baggy) collared, button up shirt. Ideally it will keep the sun off your skin, and allow air to flow between the fabric and your body. You can take it off and soak it in the pass through towns, and it will really keep you cooler and more comfortable than a jersey or t-shirt.