I have a nephew who just left on a mission for my church. He spent July 4th in a missionary training center and they probably didn't really celebrate so I sent him a fun patriotic care package.
After the training he headed to Boise, Idaho where he'll teach people about our church for two years. When missionaries are new to the mission they call them "Greenies" so I send him a green package when he arrived in Idaho. I can't just send a plain box to I wrote "greenie" all over the outside of his with different green permanent markers.
If you're looking for other missionary care package ideas check out my Fun Mail & Packages link for past ones.