Several good "hot" sauces, a favorite spice, flavored ice tea mix, couple of magazines that your friend has interest in, books that your friend likes, good pair of sun glasses, and anything your frined wants.
Alot really depends on where/what your friend is doing. Infantry=No PX Air Force= Big PX
Find out what he/she has around them and what they can get. Then send stuff they can't get.
Avoid alcohol. It dehydrates and it isn't too fun to get plastered in the middle of the desert. Plus unless everyone gets some, some one will whine to a boss/commander and your friend will be up a creek without a paddle.
Kuwait is a pretty liberal country compared to many other countries in that region, but if you are in doubt don't send something and write your friend about it.