Please remember that the park is located in the Sonoran Desert. In the summer that means very hot and dry conditions. Even in the 'cooler' seasons we run very warm!
If you have discovered you forgot an essential- a hat, sunscreen, lip balm, etc- do check in the park bookstore for such things on sale. An ounce of prevention is truly worth a pound of cure, especially as regards sunburn or the potential for skin cancer. Plan ahead!