[dsraven]

there is always the rubber or metal overlay where the step tread usually lands. a polished metal tread will likely scuff and not be as shiny where the feet land. you may want something with some anti skid surface so you don't slip off when it really matters
3m clear paint protection film but I would say it will also likely show scuff marks and becone dull where the scuffs are
if you look at aftermarket or OEM running boards or nerf bars, whatever you wanna call them, they usually have some sort of rubber product where the common foot placement lands. if done well that would likely be your best bet. you may be able to find an alternative in a marine supply store since boats are usually slippery when wet so maybe a more common and durable product could be sourced there?