If you're wearing birkenstocks in this decade then you might as well go all the way and wear socks. Otherwise put on sunblock. And if you have already been sunburned then put on aloe vera.
I swear this works. Feet sunburn, oatmeal is a lifesaver. It reduces the urge to itch, and it's anti-inflammatory properties help soothe irritated skin. There are two ways you can use it to treat sunburns: as a paste or in your tub. For the paste, grind 1/2 cup of oats in a food processor into a fine powder. Hope that helps!