nice to see there are sensible replies in this thread
Joe, is he leaving for a new job, if so get him something that could be usefull in his next job.
When I was asked at my last job what I wanted I just said I didn't know (as I never know what I want till I see it) so they just got me vouchers for various shops and I took it from there.
Do you know what he likes to do in his spare time? If so get him something related to that, if not quickly ask him (he's bound to know there is a collection so may be able to give you a bit of a hint as to what he would like or may need.