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These are already in the law, and have been superseded by lethal injection as the preferred method of carrying out capital punishment.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.
As lethal injections come under more and more scrutiny, these statutes are being reconsidered as ways to impose the death penalty and it’s probably going to be in the near future that these older methods may be used again.
4 Methods of Execution In U.S. Death Penalty Cases Other Than Lethal Injection
1. Firing Squad
Other states with firing squad as an approved method of execution: Idaho and Oklahoma.
States with electrocution (electric chair) as an execution method in their laws, while lethal injection became the preferred method of execution, are Alabama, Arkansas, Florida, Illinois, Kentucky, Oklahoma, South Carolina, Tennessee, and Virginia.
For New Hampshire and Washington, death sentences can be carried out by hanging as well as lethal injection.