The first box out to a contact from my family is usually a couple of pages typed letter describing my family, some of the things we like to do, the weather, etc. and I print up a few pages of pictures to include too. I have some standard picture sets stored in Word that I use over and over - like camping and vacation montages - with descriptions of the pictures right in the document.
After that, I tend not to write as much in the boxes - just include a short note or handwritten card. I find most of my boxes end up on the community property shelf for everyone to help themselves to anyway. By the time I get around to sending the second box, I have probably already sent more letters, cards, or postcards anyway to the contact or maybe even had email conversations with them. As I get more vacation pictures or special things my family has done, I will send them inside a letter or card that may be in the box or mailed separately.
One cool thing is that one of my contacts passed a letter to someone else and they have been writing back. Now I have been writing to that person under their own address instead of Attn: anysoldier. Kind of neat where your stuff ends up!!