What instrument did David play to bring relief to King Saul?

Answer: Harp

Whenever the distressing spirit came upon Saul, David played the harp; then Saul found relief and the spirit departed.

“And whenever the spirit from God came upon Saul, David would pick up his harp and play. Then Saul would find relief and feel better, and the spirit of distress would depart from him.”— 1 Samuel 16:23 (BSB)