Whether it is for an email or an addition to a written letter, a little humor adds something fun for your missionary to read. At the beginning of my brother's mission I tried including a joke at the end of my email, after exhausting any jokes I could find I decided a spiritual thought was a little easier. Here are a few sites I sifted through to add a little humor for my missionary to read.
http://www.mormonhaven.com/shortj.htm?vm=r
http://www.angelfire.com/az2/arizonadry/truth/jokes.html
http://www.redmondfamily.com/Monique-and-Fraser/Jokes/Mormon-Jokes/Missionary-Jokes
Here are a bunch of missionary funnies:
(click to enlarge and print)