There are several different methods of execution used by the states (as well as the federal government and the U.S. Military) that offer ways to carry out a sentence of death other than the lethal injection method.
Image: Florida’s Electric Chair
Guillotines, for example, are well known execution methods (as sadly are beheadings by other means), but the United States does not recognize this as an acceptable means of carrying out capital punishment.
Alternative Execution Methods
These are already in the law, and have been superseded by lethal injection as the preferred method of carrying out capital punishment. They have passed constitutional challenge already, these methods just haven’t been used in decades. But they’re available, statutorily.