I sympathise, Flatfish.
From my experience, women love surprises. So why not just tell her you're leaving her?
If you think that might not go down too well, ask her family and friends for tips (sisters and close friends are usually a good bet). Seeing as she has no interests at the moment, perhaps the gift could be something that could turn into a hobby - haven't the foggiest what that might be, but I'd stay away from diamonds for the time being.
Failing all that, take Hando's advice and just ask her what she wants. If she tells you that she already has everything she needs, agree with her while throwing in the 'Christmas isn't about presents, anyway' line for good measure. If that doesn't prompt her into dropping 'subtle' hints, nothing will.