A change of socks is always nice. It helps to keep your feet comfortable, because if your feet are uncomfortable, the rest of you is as well.
I have to agree with water. Even just a 12 ounce bottle is better than nothing. It ruins the fun of tromping around in the woods if you spend a bunch of it wishing you had something to drink.
A little food is nice as well. A sandwich is good for a long hike, while a piece of fruit or two is good for a shorter one. If you don't want to have to pay attention to the contents of his pack from day to day, you can make sure there's a Cliff bar or the like in there, something filling that has a pretty good shelf life.