Stuff
My son got back last year. The most popular stuff has already been mentioned but ...
Phone cards, the more the better.
Freeze dried heat proof snack stuff, jerky, nuts, trail mix, GORP etc.
Crystal Lite drink mix pre-sweetened, it can almost make a canteen taste ok.
Type up a pocket sized card with all the family phone #'s and e-mail addresses on it, then laminate it. Matter of fact, make two of them one for their shirt pocket and one for their duffel.
Steady stream of paperbacks sent two or three a month, at least.
Newspapers from home. Even though they might be two weeks or so out of date, it's always nice to see what's happening at home.
(If appropriate) Small Christmas tree, a few ornaments with a string of mini lights
Once they get there have them let you know what else they might need.
Depending on their MOS and assignment they might need anything from wetwipes (out at support bases) to the reflex sight and dust covers I "loaned" my son for his M-4 when he was pulling travel/bodyguard duty with the Colonel.
Best wishes and prayers to you and yours.