Hmmmm... There are a lot of variables in the question you are asking. Where are they going, what unit are they in, what are they doing? A finance clerk that is stationed in the IZ really doesn't need anything special. An infantryman or someone on a NPTT team living out on a COP may need more gear. At the same time, remember that soldiers are already carrying a LOT of weight what with an IBA with chest AND side plates, weapon, magazines, NODs, first aid kit, and Kevlar. And that is a minimum. You can also expect to see things like dismount radios, Camelbacks, etc on top of that.
That said, here are some things that I personally have found very useful...
GOOD GENERAL GIFTS
A Gerber or similar multitool
I use mine all the time for opening boxes, fixing stuff on my HMMWV, taking apart the AC in our room, cutting zip ties, or whatever. This is probably my number one piece of gear. If they aren't issued one, consider getting them a multitool before just about anything else, no matter what MOS they are.
A small flashlight, preferably AA
I carry my Surefire, but a lot of times I find myself using a smaller light because I don't need that much brightness and I don't want to wear out my expensive Surefire batteries. The basic AA Maglite is a great choice. Another one is an
Inova X1. This is a small, AA powered LED light. They produce a nice, even beam of light that is great for reading stuff at night, adjusting my radios, finding something I dropped or whatever. This is another good general MOS piece of gear because most of the camps and FOBs in Iraq don't have street lights (unlike Kuwait), presumably for security reasons.
A digital camera
Digital cameras are a lot of fun for just taking random snapshots of people, places, historic monuments (I have some nice pictures of the crossed sabers at Saddam's old parade field), or whatever you encounter. You don't have to worry about replacing the film, the pictures are free, and they can e-mail them or put them on a website. This was a great way to show my deployment to my family on leave.
A USB Flash drive (minimum 512MB)
This isn't really field gear. But it's extremely useful if you work in any kind of office in the Army. And even non-office soldiers will be constantly swapping music, videos, pictures, and documents between their laptops. EVERYONE has or buys a laptop downrange.
GOING OFF THE FOB GIFTS
A portable GPS (one that can display military grids!)
This is more of an Infantry/going of the FOB gift. Esp if the individual in question is an NCO or PL. A lot of times they will want to call up grids for where they are taking fire/reporting an IED/talking to an individual/need a MEDEVAC/whatever. The small, portable, wrist type GPS units are great because you can strap it to your IBA and always know where you are.
A digital watch
I suggest this solely because if you have vehicles you will need to keep your radios on PLGR time for them to sync with the other radios in the unit. I set my watch to PLGR time so I can adjust my radios or run over and take care of someone else's radios on the fly, without having to check the DAGR on my BFT. People are always borrowing my watch to fix their radios.
I guess I didn't give you too many real "field gear" ideas. That is partly because we already carry so much gear and I personally am biased against adding too much more weight to it all. And, in fairness, the Army has started issuing a lot of things that you used to see on "must have gear" lists, like gloves, goggles, knee pads, Camelbacks, etc.
The biggest thing I can say is continue to support them once they get in country. They will have a better idea of what they need once they get their. For example, I asked my mom to send my some Q-tips in Kuwait because my weapon was dirty from the ranges we did and the PX at Buehring didn't have Q-tips.
Also, you can be a huge support by sending them monthly care packages. Every month send them a box with homemade treats, favorite snacks, magazines, books, movies, whatever. My mom, bless her heart, sends my a box very faithfully every month. I got a bunch of boxes when I first got here and around Christmas, but she is the only one who has kept sending me stuff. That little taste of home is a huge blessing.