Pain, Suffering, and Medicine
People get desperate when they suffer. Diseases are everywhere. You can't develop modern medicine without modern technology. A parent watching their child suffer and die painfully is (thankfully) considered a rare occurrence in our modern world. In fact, I'd encourage you to look more into child mortality rates - the number of children who die before five years of age was around 30% in much of the world, and as high as 50% in many places until the late 1800s - and that's crazy!
Pain and suffering are motivators - such a civilization (especially one with more ability to absorb / experiment) - might be willing to travel farther and explore further into unknown regions, or to invest more time into thinking beyond stone-age technology out of the desire for alleviate the suffering of loved ones - especially children.
From limited resources, to simple things like plumbing (the toilet has saved many lives), to medicine, to even ultrasounds and c-sections, there would be great emotional desire to reduce suffering.
@Alex2006 gives great points here so I won't duplicate him: curiosity, appreciation, arts.
But even ideas around controlling temperature (A/C units), making certain specialties easier (the printing press), or automation (computers) would increase comforts.
What could occur on it's own, but almost certainly would occur as an extension of the above points (lower child deaths, less suffering, better comforts), would inevitably lead to a denser population.
As populations get closer together, the opportunity for specialties rises. As well as the needs of the population. Traditional farming only grows so much - the first industrial revolution changed that, and then again the second! The need to optimize farming for more people in a smaller area requires people to think more about issues not previously thought about - can we make food grow faster/bigger? Can we grow food inside a building? Can we make buildings more than two stories high (that will require more than simple mud-and-straw bricks).
Structural issues, transportation, food, policing tactics, cultural reforms, etc, are all issues that can arise from denser populations, and would encourage the society to advance technologically.
Threat From (Future) Advanced Civilizations
Much to your own point of outside threats, no matter how sustainable a civilization is, there will always be the possibility of a greater civilization. Even if your society is not currently being addressed by an outside threat, there can / will be an outside threat at some point. If your society is aware enough that there might maybe someday be an outside threat, you don't want to wait to meet them to begin developing technology. This is like meeting tanks with arrows, or atomic bombs with tanks, or UFOs with missiles. The possibility of an enemy is a great motivator.
We see this even in the past hundred years with the advent of atomic weaponry - once we understood that greater weapons were theoretically possible, we began building technology not around what our enemies have today, but what we think they might possibly have tomorrow - that's a far scarier (and greater challenge for progress) than ever before. Even if a society is "stable" economically, socially, religiously, etc, an enemy with an atomic weapon could wipe you out instantly. Simply put, the only limit to a theoretical future threat is your imagination - not the (disease/weapons/technology/people/creatures) which exist now. No matter how advanced you get evolution-wise, the technological capacity of humankind significantly out-grows the capacity of humankind on individual biological basis.
Although your society might be able to "absorb" a metal, rock, log, etc - this necessitates that the thing they acquire already exists. This is problematic. The creation of atomic weaponry (to stick to the example), necessitated the technological combining of many different rare elements (like plutonium and uranium) - and acquiring these resources means traveling great distances and transporting sensitive material great distances. Then, of course, comes the issue that radiation is dangerous to most forms of life, and is obviously something your creatures would not want to absorb without very costly consequences.
So the idea of a potential future threat would drive technological progress.