As others have stated, what to send depends on where your folks are at. If they are in a built up area, de-emphasize the toiletries. If they are in the middle of nowhere with very little re-supply, then the toiletries will be appreciated. Especially travel sized ones since the guys can take those out on missions. Find out if there are women in the unit. If there are, small women's items like hair conditioner and female-themed books and magazines are GREATLY appreciated, since these things are just not stocked in great abundance.
If you do send soap, double bag each bar in a ziplock bag. Same with any liquid or semiliquids. The boxes are kicked around a bit on the way over so don't send anything expensive or fragile. Don't send chocolate between March and October. It will melt. Homemade cookies are nice, but honestly, a lot of them end up stale by the time they get there. If you want to send something homemade, make a really good Chex Mix. That will stand up better. For things worn, choose subdued/tactical colors. It's fine to send a few religious items, as some of the guys will likely find comfort in them, but don't overdo it. For comfort items like nerf balls or mini pillows, colors are okay. It's a little touch of cheer when everything around you is the exact same shade of sand. A personal letter and photo is a nice touch. A lot of times, we tried to write people back, especially if their kids helped with the care package.
Here's some suggestions:
Bandanas or Buff brand neck gaiter
Coffee - instant is good if they are remote, otherwise ground
Socks (athletic white, or green or black boot socks)
Flip flops - they ALWAYS run out of "normal" sizes at the PX
Magazines - Maxim and Popular Science are good choices
Tuna snack kits
Batteries - AA or AAA
Scenic postcards or small posters
Ballpoint pens and sharpies
Dice or playing cards
Small cheap electronic games like Tetris or Texas Hold 'Em
Q-tips/ Weapons cleaning patches
Stress/squeeze or high bounce balls
Trail mix or sunflower seeds
Sunscreen - single packets are good
Ha, that's quite enough to start. And on behalf of someone who was the recipient of many many care packages in my day, THANKS.