I would retype a few jokes in the letters I wrote to my son. He told me later that he would share them with others.

Please for your son's sake, don't put anything in your letters.  Stick to the guidelines which are a plain envelope and standard writing paper. 

I will add just a little bit to your other responses - if you want to send puzzles or crossword, copy them onto typing paper and write your letter around them.  What you don't want is for anyone to be able to "feel" anything extra in your envelopes.  As long as all your letter and extra are on regular computer paper no one will know what all is your "letter".  I would send my daughter little clip art pictures to make her laugh at some of the crazy stuff she had to deal with.  Like this little "ruler guy" picture I found online when she was going through RLP and everything was measured by the drill instructors!!!!  Gave her something funny to think of while dealing with all the BS!!!!!!  Good Luck to your son!!!