Booze of some form. If he is a beer monster lots of beer, if not a decent bottle of wine.
Or if he has DC headset a new padded head band.
Or take him out for a decent meal.
And a daft thing some might say but make up a certificate on your computer along the lines this is to show that CFI xxxxx has gone beyond the call of duty of Instructing by getting xxxxx through from zero to hero without him crashing.
To be honest on a FI's wages the best gift I ever got was a box of frozen meat off a farmer student with 3 huge bags of frozen frys. It must of been home kill Aberdeen Angus beef, it was bloody delicious.