A watch that will withstand the G-shock that Marines all plop their aircraft on the gound with. -3000 VVI, maybe?
Seriously, though, I'd get something with slick wings on it. Real gold wings made by a jeweler to wear on formal uniform, engraved on back with date pinned, or collector beer mug, etc. Look at the NEX (Navy/MC version of PX/BX) web site, I bet they have good gift ideas. If leather jacket had not been issue for AF later in my career, I would have bought one for myself. I don't know about Marine uni's, just throwing that out there.
Some books about leadership and duty being above and beyond your specialty in the service.
And also seriously, pilots especially young ones, always love watches and sunglasses. Congratulations