My hubby is into guitars. He appreciated two magazines in particular: Premier Guitar and Acoustic Guitar. He subscribes to both and looks forward to them. You might mail one hard copy as a gift and tell them a subscription is coming. Or a gift card to a music store near him that he enjoys. I'd send a pack of D'Adario strings and write something corny such as "I know you actually do like gifts with strings attached!" Or a pillow case with a guitar motif.