You’re standing at the rental car counter in Denver. Your flight landed two hours late. The agent asks for your driver’s license. You pat your back pocket — nothing. You check your carry-on. Empty. That sinking feeling hits: you left your wallet on the kitchen counter back in Chicago. Domestic travel feels low-risk. You don’t need a passport. You’re just flying to another state, not another country. But that casual attitude is exactly why travelers get stuck without the right documents. Here are the six documents you should never leave home without, even for a trip inside your own country.…