Suit-less survivable atmosphere

• Pressure suit-less survivable
• Warming suit-less survivable

Mars

For the human body the pressure of the atmosphere is the same as a vacuum.
So it's not survivable (beyond a few very unpleasant seconds as is a true vacuum too).

Upping pressure to make it survivale would amount to a minimal variant of terraforming.
Splitting rock to up the oxygen pressure to the same partial pressure we have here on Earth.
Maybe reducing CO2 levels to a bit lower partial pressures (small problem in comparison to upping oxygen).
Nitrogen reserves ion Mars not very clear as of 2025.
Without the nitrogen flammability might be a bigger concern.

One cannot run out of oxygen as all the stones are made partly from it.
And strip-off-rates by solar wind can easily be replenished.
The issue it getting all the oxygen in the first place.
A centuries long project even with most advanced technology currently imaginable.

Venus

At 50km height pressure and temerature are already survivable without a suit.
Breating mask needed as 100% CO2 is rather bad to breath in.
If the concentration of the sulfuric acid in the clouds at that height is low enough
(it might be as it is very dry there) then bare skin might be tolerable for a while.

Even just removing the sulfur from the surface would be an enormous project.
But it might make the skies clear and exposed skin safe.
Unclear in how far longer term exposure of all of ones skin to 100% CO2 is problematic.

Titan

Very nice nitrogen atmosphere of 1.5atm. But very cold.
There are cold chambers of -100°C as medical treatment, Titan is quite a bit colder still.
In fact just barely above the condensation point to liquid nitrogen.

• Good: Slow wind-speeds may make the situation better
• Bad: The low temperatures lead to a high gas density at 1.5 atm and thus to better heat conductivity.

=> Unclear if and how long exposure of bare skin to titans air is safe.
It seems there can't be much done. Heating the entire moon by a lot is likely not possible.

Water worlds

Ceres, Enceladus, Titan, Callisto, Ganymede, (Europa), Triton, …

In water filled underground caverns the water could be considered the atmosphere.
This would pretty much amount to ice-diving.
Brine can even be quite few degrees below 0°C which might be quite problematic as
it may nigh instantly freezing any exposed skin.
Also the extraterrestrial water might contain a lot of ammonia NH₃
which would be rather unhealthy both …

• for the skin directly and
• for the body due to diffusion through the skin.
  

Gas/Ice giants

For the ones with nice gravity (all but Jupiter have 1g)
it may be too cold at depths where the pressure is not yet too high.
Ignoring the challenge of human survivable landing a spaceship
onto the 1atm level of a gas giant transitioning into plane flying mode.

Related

• Colonization of the solar system
• Colonization of asteroids

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• Underground transport and habitation