If your son is headed off to an overnight camp, then you want to write letters to let him know you are still thinking of him while he is away. One tip is to start a drawer or pile of clippings you collect a few weeks before he leaves for camp. Then you will have a stash of material to send as soon as he leaves.
Though the world is connected with smart phones and the wireless internet, the camp your child is at may have limited internet access, or prefer the campers not use those devices unless there is an emergency. Include the sports scores of his favorite team. Additionally, you can include newspaper and magazine articles about his favorite team that he can enjoy and share with his friends while at camp.
If your daughter loves to solve puzzles such as crossword puzzles or Sodoku, then you can create a small crossword puzzle or Sodoku game in the body of the letter. Another option is to clip these games from your local paper. You can even start the puzzle for your daughter, and she can finish it at camp so the two if you will have finished it together.
As much as your son might complain about his siblings, the truth is he misses both his parents and his siblings while he is away at camp. Include at least a sentence about each member of the family. It might be something about an accomplishment that a member of the family has had, or something simple like what the family watched on television together. Mentioning the other members of the family will help keep your child connected to your family while he is away. And, don't forget to throw a line about the family pets.
If your daughter loves a good joke, then write out a couple of jokes for her to enjoy. Even if you are not a master comedian, you can find jokes online, or printed daily in your local paper. She can then share the jokes with her friends. Just make sure the jokes are appropriate and can be shared in a general audience. A variation on sending jokes is sending comic strips of her favorite comic. They can be cut out of the daily paper and included with the camp letter.