I am on my second missionary and have found these ideas to send to a missionary you know. You want your missionary to feel loved and you have most likely sent goodies and treats (or plan to ) but you will love the simplicity of these ideas during this busy time. I am not affiliated with any company or product and so these opinions and links are purely to help anyone interested.
1. Packing Cubes! My favorite item is so useful even if your missionary is headed home soon. Light-weight and useful after a mission when your son or daughter is headed to college. Each of my children left with a set but said they wished they had more. Your missionary can keep everything organized and with a zip be ready for transfers. Love the selection of colors too!
2. These scripture notes are transparent pages for inserting notes into your scriptures. These are extremely useful when your missionary wants to write extra thoughts or notes from zone conferences or personal study. I found them by the scripture markers at Deseret Book.
3. FlashDrive with new mission approved music. Both missionaries have mentioned that they get tired of the music they take on their mission and love getting new songs. I would buy some new music and copy it to a flash drive. I did learn that each missionary apartment has a dvd player from the church for watching training dvd's and they have a usb port so that a flash drive can be listened to. This was even true in my daughter's mission in South America - if out of the US just be aware if items are easily stolen. A flash drive would be easy to slip into something small if you think it will make through the mail service.
4. Family time on a flash drive. This goes along with the last suggestion. You can now record on your computer or iPhone your family singing or talking to your missionary and send on a flash drive.
5. P-day t-shirt. My son is spending a ton of time doing service - painting, moving people, and other hard work. Nice for him to have a new shirt to play and hang out in on P-day.
6. Shout wipes - You would think your missionary would hate getting stuff like this but my son says he is always spilling on his white shirts (usually red sauce) and is so grateful to keep these in his bag.
7. Awesome stamps - my Elder was stoked to get Batman stamps recently -- he was running out and thought they were fun. Check your post office to see what fun stamps they have that would match your missionary. My son doesn't write a ton but he does drop notes to thank ward members or family who have sent him a letter or package.
8. From the Elders/From the Sisters -- there are awesome notecards and sticky notes that help missionaries leave a note for an investigator that forgot about their appointment. These are found at Deseret Book. If you don't live close to a Deseret Book some of these can be found online at their website.
9. A new tie for an Elder and a scarf or earrings for a Sister. It may seem unoriginal but missionaries get tired of what they have and like to switch it up. Ties and scarves are where they can add a little color and personality.
10. A small album of more family pictures and inspirational quotes - they can't ever get enough pictures!
11. New recipes -- they forget what to make and love having a new idea for the next time they shop.
12. A Frisbee or Nerf boomerang - a great p-day activity for all personality types.
13. Stickers and CTR rings to hand out to children in your missionaries area.
I hope you found an idea you can use for your missionary. Christmas is a wonderful holiday but it is even better when you know someone sharing the real meaning of Christmas with the world! Be sure to share your great missionary christmas ideas in the comments below.