I know many presidents take more indirect roles, running political foundations and other organizations, but has there ever been a president who, after their term concluded, became a senator, governor, or other 'lower ranking' political official?
John Quincy Adams was president 1825-1829. He ran for and won a House seat in 1830. Adams served in the House for 18 years.
Andrew Johnson was president until 1869. He returned to the Senate in 1875. He died a few months after taking office.