Said like that sounds really simple! Being mindful is a fundamental part of the practice. But it's deceptively simple.
The problem lies in the fact that "just" doing something, whatever it is, has much more to it than what meets the eye.
Let me just give you an example: you're "just" reading these words right now. Are you sure the only thing that is going on is "just" reading? Are there thoughts in the back of your mind? If so, do they have the power of completely stealing your focus? If they don't have such power, are they more or less like noise in the radio when the station is poorly tuned? Or are just like persistent shadows in the background, barely noticeable? And how will it be dealt, if dealt at all? You won't pay attention, you'll block them, you'll relax a bit, you'll sharp the focus, you'll observe without involvement? And how will it be after an entire day trying to do it? Will you exhaust yourself, will you gather tension and so on? In fact, what is blocking your ability of just doing whatever you do? Is it only one kind of obstacle, different kinds, how are they overcome? Is it this all there is to it? Or do we also need concentration? And if so, why?
You just do it, a little like in the Nike slogan, but it takes a lot to learn how to "just do" something.
Sitting is simple. Just sitting is not, even when it seems so.
Most of all, simply being mindful of trivialities doesn't cut it. Mindfulness directs the power of concentration. Concentration brings about the power by which mindfulness can penetrate deeper and deeper in the levels of mind. Balancing concentration and mindfulness leads to insight and understanding. You see, concentration and mindfulness are distinctly different functions. Both are needed in meditation and achieving a balance between the two is almost an art. Too much awareness without calm, achieved bv the practice of concentration, results in a wildly over sensitized state similar to the trip caused by acids. Too much concentration without a balancing ratio of awareness will result in a sort of stiffness of the mind. One sits and stays, absorbed in meditation, but in fact one goes nowhere beyond such state (that some mistakenly take as enlightenment, while in fact they are damaging their intelligence).
It's good to have some training conditions to develop concentration and learn the skill of mindfulness. Once learned, and this is trickier than most think, one can dispense with the training restrictions. One doesn't even need to be in a calm state to be mindful. I've said something along those lines in another thread. Sometimes you need to catch the mind in the act when its behaving like a monkey. One can be mindful while solving problems which imply hard thinking and one can be mindful at a party. One can be mindful when one's mind is affected by the five poisons, even when fury is raging! But we won't get to such point by just sitting or just doing whatever we do without knowing a little more than that.
That's the main point I'm trying to make. To be able to "just" do something implies knowing quite a bit about the method, meaning and roles of mindfulness and concentration.
The slogan "just"[insert whatever you want], is appealing, but without further information becomes a decoy. One bites and gets even more stuck.
I know that wasn't your point, but as you've asked for our thoughts... I just did it!