I’m looking for an adjective that describes a person who appreciates the little things in life: someone who takes life at a little bit slower a pace, who enjoys being outside instead of forever inside, and so on and so forth.
Consider the term aficionado (“A person who likes, knows about, and appreciates a particular interest or activity; a fan or devotee”), for use in a phrase like “She's an aficionado of life.” I don't know of an adjectival form.
The verb savor (“to appreciate, enjoy or relish something”) is relevant, eg “She savors life.” Again, I don't know of an adjectival form.
The adjective appreciative (“Showing appreciation or gratitude”) may apply, but is somewhat nondescript and general. Optimistic, positive, and upbeat (“Having a positive, lively, or perky tone, attitude, etc.”), are perhaps more relevant; a sentence like “Sally's an upbeat person” implies Sally is an active person, appreciative of life, has positive attitude, etc.
The adjectives epicurean (“pursuing pleasure, especially in reference to food or comfort”) and hedonistic (“Devoted to pleasure; epicurean”) may apply but are slightly pejorative. (Note, hedonistic is antonymic to ascetic (“...characterized by rigorous self-denial or self-discipline; austere; abstinent; involving a withholding of physical pleasure”).)
You could try equable: (of a person) not easily disturbed or angered, or tranquil: free from disturbance; calm. Another option would be transcendentalist, although that may be muddying the waters a bit.
This might not be quite what you're looking for but you might like flâneur (It's a word that I'm personally very fond of). Specifically it refers to a person who walks through public spaces, watching and appreciating the world going by. You can use it in a broader sense to refer to a person who views life this way (i.e. not necessarily walking around). However it does have a strong suggestion of at least being in the same area as other people, you might be pushing it a bit to use it to describe someone who enjoys being outside in areas without people.
There's more information here, it seems like you'd be using it in the "6 Other uses of the flâneur" sense http://en.wikipedia.org/wiki/Fl%C3%A2neur#Other_uses_of_the_fl.C3.A2neur
Let me make sure I understand you: you’re looking for someone who takes the time to walk thoughtfully in the park or to take in the untrammelled wilderness where he can stop and smell the roses instead of being forever caught up in the maddening hustle-bustle of the frenetic urban pace that has come to completely dominate to the point of dangerous affliction far too much of modern so-called “society?
If that is the case, perhaps the best term for this is simply uncommonly sane.
Then again, that doesn’t say why. Depending on just where you you would care to place your emphasis, other related terms might include various and sundry selections from the following:
happy, healthy, uplifting, pleased, pleasant, wise, temperate, steady, irenic, laid-back, easygoing, poised, unflappable, untroubled, peaceful, serene, placid, relaxing, imperturbable, measured, halcyon, bucolic, enchanted, charmed, tranquil, staid, calm, unruffled, peace-loving, untroubled, composed, discerning, careful, alive, enlightened, joyful, perspicacious, sophic, alert, percipient, sagacious, insightful, childlike, wonder-filled.