Pathfinder has rules for low gravity:
Low-gravity areas, such as those in which the effects of multiple graviton-based artifacts interact in unpredictable ways, are PC playgrounds, in which characters’ relatively hyper-developed muscles are far more effective than normal.
In an area with only a third of standard gravity, for example, PCs can jump three times as high and as far and lift three times as much. (Movement speed, however, stays the same, as moving in great bounds can be awkward and difficult to control.) Projectiles have their range categories tripled.
The moon’s gravity is a sixth of Earth’s, so for the moon it would be jumping six times as high and as far, lifting six times as much, projectiles having range multiplied by six, and so on.
Contrary to popular conception (and the only existing Pathfinder rules on the subject, the starsoul sorcerer bloodline; thanks ShadowKras for finding that one), space is not cold. Frost would form as any moisture on the skin rapidly evaporates (which, of course, cools the surface it evaporates from), but the damage of that would be minimal.
After that, the concept of “temperature” is kind of difficult to apply to space, but the long and short of it would be that someone on the moon would effectively have exceptionally good thermal insulation. Without an atmosphere, there is no convection on the moon and conduction would only be through the feet to the lunar surface—which is a pretty poor conductor itself.
But their body would still produce heat. And if they were standing in direct sunlight, they would also be absorbing quite a lot of heat, since relatively slow or not, the sun is a ridiculously colossal heat source and its radiation is intense. So heat, rather than cold, is the primary concern.
The exception may be the dark side of the moon. On the dark side of the moon, people are standing on extremely cold lunar dust (−250°F), but it is a poor conductor of heat and that is only a small point of contact, so it may not offset the heat produced by their bodies—it is conceivable that they would get frostbite while also suffering from heat stroke!
(For reference, the Apollo missions avoided all this by scheduling all extra-vehicular activity at lunar dawn, and of course by having highly reflective and insulated spacesuits. I have not been able to find much in the way of convincing data on what would happen if standing on the moon bare foot.)
Pathfinder has rules for heat dangers, but they don’t apply very well to the moon. Even the rules for frostbite are pretty limited, so only “A character who takes any nonlethal damage from cold or exposure is beset by frostbite or hypothermia (treat her as fatigued).”
Work in Progress
The moon lacks a magnetic field, which is the primary protection a planet extends against radiation for its inhabitants. Pathfinder has rules for radiation, but they annoyingly lack any good way to determine if something is a low, medium, high, or severe danger. But I would go with low here—being out in space is not the same as sitting next to an unshielded nuclear reactor or something, and the damage caused by the radiation would, realistically, be a very long-term thing. Acute radiation poisoning does not seem to be a risk of exposure to outer space.
Moon dust sucks
This was actually one of the more surprising scientific discoveries by the Apollo missions: the stuff gets everywhere, and sticks to things surprisingly well.
It’s also really abrasive.
The dust was so abrasive that it actually wore through three layers of Kevlar-like material on Jack [Schmitt’s] boot.
– Professor Larry Taylor, Director of the Planetary Geosciences Institute, University of Tennessee (2008), quoted here
So if your unwitting lunar explorers are barefoot, or even wearing regular shoes, damage to the feet is quite likely. Caltrop rules are appropriate here.
On top of that, super-fine, sharp, abrasive material that gets everywhere? You do not want that in your eyes or lungs. Breathing in moon dust would be a serious hazard, getting it in your eyes would cause serious irritation at the very least. So if some magic effect allows them to breathe, but doesn’t cover their mouths, the dust itself could easily interfere with breathing. Without eye protection, blindness is potential hazard. And low gravity makes it surprisingly easy for it to get places, even in the absence of atmosphere.
Work in Progress
The moon has no atmosphere. Pathfinder has rules for suffocating due to lack of oxygen, but those rules still presume a pressurized environment of some sort, which the moon isn’t.
The effects of vacuums on the human body have been grossly overstated by media; you do not explode nor does your blood boil if subjected to a vacuum. However, the results are still quite unpleasant.
To begin with, you cannot hold your breath in a vacuum without some kind of pressurized breathing apparatus (e.g. oxygen mask). Moreover, the reduced ambient pressure can actually force oxygen out of a person’s blood, causing rapid progression to hypoxia. In game terms, that amounts to applying the suffocation rules immediately, rather than after many rounds and a failed Constitution check:
In the first round, she falls unconscious (0 hit points). In the following round, she drops to –1 hit points and is dying. In the third round, she suffocates.
That results in 18 seconds before death. The reality is not that severe (estimates as high as 90 seconds exist), but this is more consistent with Pathfinder rules.
Additionally, attempting to hold one’s breath when subjected to vacuum could badly damage lung and throat tissues. If someone is exposed to vacuum very suddenly (decompression from 1 atmosphere to 0 in under half a second), it may be physically impossible to safely expel air from one’s lungs fast enough to avoid this. At slower time frames on the order of a few seconds, I would probably call for a Reflex save around DC 15-ish—not terribly hard by PC adventurer standards, but not automatic, either, and you only get to attempt the save if your character knows enough to exhale immediately (attempting to pin a Knowledge DC on this would be far too campaign-specific for me to venture a guess here, I think).
In terms of turning that damage into rules, I think attempting to describe this damage in HP terms is kind of missing the point: this would be a death sentence on a very short time limit. I allow (any) magical healing to fix the damage, but it would have to be applied before that person’s third turn after being subjected to the vacuum, even if pressure was restored.
Considering how quickly suffocation occurs, without an oxygen mask we can probably stop considering the effects of the moon’s environment there.
But if we have a pressurized oxygen mask (or magical replacement of some sort), but no pressurized space suit, then the other effects of vacuum also apply. Human skin and blood vessels are tough enough to maintain structural stability and prevent blood boiling, but there is still paralyzingly-painful swelling and expansion of blood vessels can cause a dangerous drop in blood pressure. Moreover, the formation of bubbles in the vessels can be extremely dangerous (decompression sickness or the bends).
To turn those into game terms, I say that anyone subjected to vacuum without any kind of pressure protection, must make a stiff Fortitude save—DC 25 seems appropriate—or else be staggered by the pain, swelling, and/or decompression sickness (which might be generous, paralysis might be more appropriate, but I prefer to still give someone in such situations a chance to do something about it). If such a person also has some way to breathe despite the vacuum (or doesn’t need to), they could operate in this way for some minutes.
As for what happens if someone with an oxygen mask but no other protection from the vacuum continues to operate for extended periods of time, so far as I can tell, the human race does not know. That situation has never happened to anyone, and no one’s volunteering to try. Personally, I just leave it at periodically rerolling Fortitude saves against staggering every minute.