My little brother left the MTC and entered the mission field recently. Right after he wrote home the first time telling us about his area and companion I put together this "Treat your trainer" package.
The idea is that although missionaries get a lot of packages when they first go out, the packages slow down considerably most of the time the longer they've been out. This package was meant to be exciting for my brother's companion and my brother as well.
I packed it all into one of those flat rate boxes and decorated the box with the Elders' names and "Treat your trainer" so he'd get the theme of it.
Air freshener plug ins: Treat your trainer to a great smelling apartment!
(My brother mentioned their apt smelled weird when they moved in)
"Treat your trainer to some candy! Some for him and some for you"
(I got 2 boxes of each kind)
"Treat your trainer by making him dinner"
I included stuff for Suddenly Salad and chicken salad sandwiches - one of his favorite easy meals.
"Treat your trainer to some cool treats! Take a break from the heat & cool off with a popsicle"
These Fla-Vor-Ice popsicles are perfect to send in the mail because they can be left at room temperature and it's HOT where he is.
Finally, I looked up a restaurant that was close to his apartment since he doesn't always have access to a car and included a gift card (enough for him and his companion to eat).
"Treat your trainer to a night out to eat"