We do a huge project each november, for the past 6 years. We have sent over 400 care packages.
If they are in the desert they like: lint rollers, baggies to store stuff in, chapstick, drink powders, sunscreen, lotion and bug spray. Camping pillows, pillow cases, boxers/pj bottoms, under armor, socks, tshirts-any extra clothing. They also like winter gear, as it gets cold there at night.
Good snacks: pretzels, jerkey, beef sticks, cheese and crakers kits, nuts, raisins, m & m, oreo cookies, doritos....endless list. (We always say "think like a 13 year old kid" and buy what they would love to eat. Usually try to get it individidually wrapped so it stays fresher. I try to send stuff that has some protein so they have quick pick me ups when they are out on duty.
I have mailed soda, just packed it in bubble wrap and put it in the center of the box.
If I liquids, I put them in a baggie before putting them in the box.
We use the APO/AFO boxes, they are larger and prepriced, no matter how much they weigh.
Last year we sent movie night in a box, complete with pjs, snacks, movies and such. Lots of fun.
PS-pay attention to custom rules and regulations. Some countries do not allow certain items in, and will open the package and 1. remove the item, or 2. return the package to you or 3. keep it. I have had all three happen.
I always send books of stamps. They soldiers have to buy them, and yes they have to send stuff with stamps. They do get to send it at the US rate instead of sending stuff at the overseas rate. Always send stamps and cards...hard to get stuff printed in English.