Careful flattery of the player works well as a valid game reward. Focus on making the player feel good, it’s a game, make them feel good and they will play again.
Flattery is the act of giving excessive compliments, generally for the purpose of ingratiating oneself with the subject.
“Flattery is telling the other person precisely what he thinks about himself” Dale Carnegie quote
“The aim of flattery is to soothe and encourage us by assuring us of the truth of an opinion we have already formed about ourselves.” Dame Edith Sitwell quote
Excessive, overly false and repetitive flattery will become tiresome and transparent.
Careful flattery could be called a compliment, which might be ‘better’ than flattery, although risks being too subtle for gamers in the heat of the action, where most activities are very unsubtle.
Compliment an expression of praise, commendation, or admiration: A sincere compliment boosts one’s morale.
Plutarch recognized that flattery, “which blends itself with every emotion, every moment, need and habit, is hard to separate from friendship.”
quoted from “In Praise of Flattery” By Willis Goth Regier
Carefull flattery should be focused on helping the player achieve their goals, which is likely to be to feel good and enjoy themselves.