I was in the Army for five years. I went to a lot of bases and was inside many buildings.
A good description of any military building is that they are built out of solid stone bricks and painted grey.
The only time you really encounter the wooden billets is in like basic training or national guard bases- the stuff was built during like WW II. Some bases still have some billets like that, but they are rare to encounter these days. Watch the first part of the film Full Metal Jacket. These kind of buildings aren't common anymore and when you do encounter them, they aren't very well kept.
Regardless of which type, the kind of things you can expect are meticulously polished tile floors not unlike you would find in a hospital. There is a huge obsession with mopping and buffing floors, and not on a "once a week basis", but a once every couple hours basis. Despite the obsession with floor cleanliness, the plumbing and electrical wiring of buildings leaves a great deal to be desired.
You can expect most buildings to have a firewatch (basically two lower enlisted soldiers who sit there to answer phones and make sure crazy people don't run into the building to kill everyone) at the front doors.
Most all modern billets (the ones made of stone) are connected to a mess hall (aka cafeteria). There are also many laundry machines (and behind these machines are probably the dirtiest areas in the entire building, yet they rarely get cleaned) on the floors soldiers live on and these floors are pretty much like single bedroom apartments with huge portable closets (lockers) dividing up the room in 2 areas. There is nearly always 2 soldiers to a room. Every room has one bathroom, about 2 windows, and no kitchen. There is no carpet, only the tile floors so many soldiers get rugs.
The offices of units are usually built into the bottom levels of the buildings the enlisted soldiers live on. The office also contains the supply depo (backup of military gear, and sensitive items like weapons, ammo and night vision goggles that soldiers aren't allowed to keep in their rooms). The supply area I would best describe as a chicken-caged area in a room about the size of a shopping mall martial art dojo, and inside the caged area is a desk for the supply sgt and shelves of card board boxes where gear is kept, and a locked vault where weapons, ammo and night vision goggles are kept. It does not look like Q's room in James Bond, as most military films depict it. It looks more like the warehouse for a military surplus store, only it can be anywhere from the size of a bedroom to the size of a small dining hall depending on if the unit is infantry or not (infantry unit generally means a bigger room than say a logistics unit, but this is not an absolute).
This is one reason why most films that depict attacks on military bases are unrealistic since the majority of soldiers have their weapons locked up in a vault that generally speaking only 3 people in their unit can open, so the majority of soldiers on a base aren't gonna do much fighting for the first few hours until those vaults get opened up. If an attack went off in the middle of the night, chances are good nobody in the damn building can even open the vault since many upper enlisted with the keys don't live in the building (they are married and may not even live on base). Soldiers are not allowed to keep their own firearms in their rooms, and if your CO is a particularly jackass individual you can't even have so much as a bowie knife or a paintball gun.
So on that note, I wouldn't worry about possibly depicting a base as being unprepared for an attack because that would be closer to reality than you may think. When I was deployed, in our commander's infinite wisdom our weapons were kept at one end of the camp locked in a connex while we slept in tents at the other end of the base. It would take at least 2 or 3 minutes for me to run across the camp. I'm absolutely serious about this.
Billets do not tend to have air conditioning. Soldiers tend to prop fans in their windows during the summer, but in my experience it does little to change the fact you are in a brick building and bake accordingly. They do have gas heaters for the winter.
There is normally a grass field directly behind the building where soldiers have morning physical training (think of gym class from high school, only at 4 am in the morning with a lot of yelling going on since everything is done in cadence).
There are also buildings where no soldiers live and exist only for things like discharging soldiers, legal aid, receiving new soldiers and getting medical shots. These are basically like you'd expect to see in any huge office building and resemble a meps station somewhat. There are normally meps stations local and if you really wanted to you could walk inside one and look around to see what I mean.
There are buildings for storing vehicles, but most vehicles not under repair are stored inside marshaling yards. These look identical to the kind you find outside national guard armories, only much larger. Each marshaling yard belongs to a specific unit, so you don't see tractor trailers in the same yards as tanks very often since those are different jobs. Each marshaling yard is connected to a big brick building that houses the maintenance (mechanics) garage and offices. It looks pretty much like any civilian mechanic garage.
Hospitals resemble any veteran hospital you've ever seen. There might be a library of donated books, there are usually televisions everywhere in the waiting room turned to some news channel, and there definitely isn't a gift shop.
Shooting ranges are generally the same ones used during WW II or Vietnam, depending on when the base was built.
Most military bases have so much space surrounding them that the government leases property to local farmers. It's not unusual for dairy cows to be out around where infantry conduct field exercises.
I think that is about as best description as I can give.
