Rose,
When my husband was serving in a combat unit in the 1st Gulf War, I sent him candy (hard candy, individually wrapped so it wouldn't melt), bug repellent (lotion kind, not spray), socks (he wanted wool socks, said they were better), sunscreen, paperback books. Generally things that were light to carry and wouldn't spoil or melt. Home baked things such as cookies were always appreciated. Just make sure they are well wrapped so they won't go stale.