Had another thought. Musicians tend to be fascinated with instruments - even ones they don't play. (See the old thread about how many instruments we all have in our houses!) Some of my S's favorite gifts have been off-beat instrments - the clay ocarina, or the djembe (drum). He loves harmonicas, but that probably a bit too commonplace!
And if you could ask your D exactly what instrument he plays, there are some nice accessories available - if he plays sax, for instance, a nice reed case, or strap. More generic - a nice metronome. The kind with the swinging arm (mechanical, versus electronic) are expensive, but nice.