Let’s take an untemplated human who’s low on hit points. Let’s also say that, for whatever reason, they don’t have access to spellcasting, SLAs, supernatural abilities, magic items, psionics, or any class levels or feats that grant any of those things (so if there’s something that can help as a 10th level feature, but the class grants a Su ability at 9th level, that doesn’t count).
What’s the fastest way for them to heal an arbitrary number of hit points, only possessing/using entirely mundane (extraordinary) features and feats and not using any benefits tied to certain locations?
The standard method is to take a couple days of rest, regaining up to 2 hit points per level per day (4 with long-term care). Not bad, but we can probably do better.
The 6th-level Iron Heart maneuver Iron Heart Endurance takes a swift action and heals them for twice their level in hit points, but it only works until they have a bit over half their hit points. For a couple well-timed warblade levels, they can use this every other turn to get a bit above half health. That’s a start, but we need a way to fill in the rest.
There’s also the well-known Troll Blooded feat, but that’s Dragon Magazine. Let’s go for only WotC-published 3.5e (or unupdated 3e) material.
They could take Combat Vigor or Vital Recovery, but both of those require an actual encounter to take place. Maneuvers may be recovered after a minute of rest (ToB 40), but both those feats are explicitly once-per-encounter. They could carry around a bag of aggressive rats, but once they enter combat with one of them, they’re no longer meeting the "out of combat" criteria quite right.
And… That’s all I can think of. What other options does an entirely mundane character have for regaining lost hit points?