"250.50 Grounding Electrode System. All grounding electrodes
as described in 250.52(A)(1) through (A)(7) that are
present at each building or structure served shall be bonded
together to form the grounding electrode system. Where
none of these grounding electrodes exist, one or more of
the grounding electrodes specified in 250.52(A)(4) through
(A)(8) shall be installed and used."
Based on the above, the ground rods are it!
You may want to consider checking the integrity of the existing rods. You may also install another of the GEs, and/or check with your local AHJ.
Based on the letter of 250.50, the rods are all that is required, lacking any other available item.