To work around the space suits are SCUBA gear problem, I was wondering if it wouldn't make more sense to just bypass the lungs and their vent hole altogether and to oxygenate blood and remove CO2 (and possibly other exotic toxins) by routing it through an external machine. Maybe something light you can strap to your back or arm.
I mean... babies do this for several months before they're born, right?
Now it's self-evident that bypassing the lungs and doing away with the pressure suit won't offer any protection from heat, cold, or radiation, and you'd have to deal with those by other means or avoid those situations.
What it does offer is protection from some range of noxious atmospheres, the removal of acute vulnerability to punctures, and the freedom to talk (atmosphere permitting) and eat and poop (universe permitting) and bathe and change your clothes and generally to stay in those environments for as long as you can find fresh oxygen and power for your oxygen machine (and it may be harvesting what it needs from the local environment anyway).
It also obviates a weakness of human lungs that they only work in a restricted pressure range. A narrower range than a human might be able to endure otherwise (not a complete vacuum, but beyond 40000 ft Earth altitude).
But maybe it's not as simple as all that. Perhaps environments that seem otherwise benign are actually substantially less survivable than it would appear. The skin provides a porous interface to the local atmosphere, for example. It may allow reactions with the environment that are unhealthy or dangerous, or the local environment may obstruct reactions which are essential.
Wandering about on Wikipedia, methane and carbon dioxide atmospheres seem not uncommon, but anywhere we know that well has other serious blockers for human habitation (radiation and cold, mostly; also, wrong gravity). If not for those other problems, would they be safe environments?