If you want to strengthen your feet for barefoot running or because you sit at a desk all day, then there are exercises out there for you. From the pencil pickup to the towel crunch.
A simple exercise that will help strengthen your feet, is walking tippy toed. Ever notice how small children seem to walk on their toes whenever possible? Seems like they know something we don’t.
Here is a good article I found for you that lists quite a few exercises. Enjoy!