I just pulled 8 metal fence posts that were sunk 2 feet in concrete a few days ago. The leftover holes are 1/2 full up with water already (ground is still wet from the rain).

What's the best thing to use to fill these holes? This the back of my garage which is all landscrape rocks so I'll never grow grass back there. Should I fill with gravel or sand? or just use fill dirt/soil? or some combo of each?

Use cheap fill dirt if there is no option to grow grass

Gravel

Coarse sand lightly tamped using shovel handle or 2x4.
Keeps from having a hole when rain settles fill material.