1. Water filter
Soldiers drink enough crappy water in the field when a lieutenant decides the platoon needs to practice using iodine tablets. While relaxing in the barracks, they need a decent filter. Pitcher filters allow the water to chill in the fridge, but faucet-mounted units provide water on demand.
No one wants to hear their roommates' music, movie, or video game, so headphones should be on everyone's list.
For a cup of coffee before PT, the DFAC is no help. Plus, coffee in the room is great for troops during lazy weekends when throwing on a hoodie and walking to chow is too much work. A cheap coffee maker allows a soldier to create their own brew on demand. For those under strict barracks policies against coffee makers, French presses aren't forbidden and are nearly as easy to use.
4. Hot plate
The DFAC won't serve grilled cheese at 2 a.m. on a Saturday morning when soldiers are craving it. Hot plates with a couple of pans allow for diner quality meals at home. For those who only grill, griddles allow for mass production or George Foreman grills are good for chicken and hot dogs. Keep an eye out for inspections however, since few commands allow hotplates. Some barracks now have stoves, so troops there can just buy pans.
5. Rugs (or a carpet for the swanky)
Rugs and carpets hide a lot of the stuff first sergeant looks for on the floor, and they grab up lots of the hair and debris that forms dust bunnies. Plus, carpet is more comfortable on sensitive toes after a long ruck march.
Of course, carpets and rugs necessitate vacuums. There's only a few hundred square feet to cover though, so a cheap vacuum is generally fine. Roombas can wait until the soldier marries out of the barracks.
7. Family photos, whether of a real family or not
Units want to see that soldiers are settling into the barracks, and that means personal items should be up on the walls. If the family is normal, the trooper can put up actual photos. For those with hot sisters and creepy roommates, a trip to a stock image site could be helpful. Just print and frame.
8. Plunger/Toilet Brush
Troops go to the field, even the human administration folks. They are fed MREs. They need a plunger, and they need a toilet brush.
9. Tons of cleaning wipes and air fresheners
Surprise inspections center on a few things. 1. Dishes in the sink. 2. Dust accumulated on ledges. 3. Smells. The dishes are simple enough. The best way to deal with dust and any odd smells is to stock up on cleaning wipes and air freshener. The wipes make cleaning the lips of door frames and the fridge easy, dealing with the dust. Air freshener helps the room at least smell clean so first sergeant will move on to the next area.
EXTRA BONUS: A hot tub.
Because that's how you win at barracks life.