I haven't sent my kids any yet, but I got a few back in the day. Usually some organization offers these, but I would advise customizing your own based on what they really like.
My mom used to send them to me...It was usually stuff like cheese and crackers, poptarts, some chips. Stuff like that, nothing that wasn't store bought. And usually like 20 dollars in cash.
Just send them what they like when they are at home. Home made is perfect if you can do that. Perhaps some personal touches to let them know you appreciate them. Oh, and some cash in the form of a small bill is nice too. Just like when they were growing up at home.