I went through an adopt a soldier program and helped some guys at a remote outpost. A lot of the stuff they liked has already been mentioned. Some food stuff I'd add is anything spicy: hot sauces, seasonings, etc. And individual packets of water flavoring, with and without caffeine.
Hygiene products of all kinds were the most requested items. Wet wipes, castile soap towelettes, high quality razors, deodorant, chapstick, lotion, body wash, etc. And anything that smells good. The men's collection at Bath and Body Works is actually pretty awesome. The scented pocket sized hand sanitizers they have now are great, and most importantly, never open and leak like some others I've carried.
If you're using a flat rate box don't let any space go to waste. You can fill voids in the box with small loose things like cough drops and hard candy.
You definitely won't hear complaints if you can get your local Little Debbie guy to donate a few cases and ship those over.