We could describe enlightenment as maybe having something to do with the degree to which one has trained the mind metaphorically back to the original mind, towards what we might refer to as non-conceptual awareness. The natural, relatively unconditioned pre-verbal state. Just the meat, the flesh, prior to conditioning. Pure, fresh, bare awareness, without much attachment to what is appearing. Everything is happening without grasping or resisting.
My tendency is to lean hard on awareness as the key. Training the mind so that awareness is happening a high percentage of time, and for long continuous stretches. In order to get that happening, I recommend structured approaches to meditation that incorporate some kind of constant interaction or feedback, as well as placing a priority on the sensate experiences of seeing, hearing and feeling. Things like (Mahasi) noting practice or breath counting are structured approaches. Less structured styles such as generic mindfulness or MBSR can work, but in my experience the average person spaces out too much if they don't have some method to keep them on point. Good practice makes good practice permanent. Practicing spacing out a fair amount of the time tends to teach the mind to space out a fair amount of the time.
It's not simply awareness that we are after, but more specifically awareness of awareness. It's that extra layer of awareness, that recursive awareness where one knows what one is aware of, that seems to make the difference. This monitoring or observing mind has a kind of freedom within that extra distance or space, in contrast to a mind that is deeply embedded in and attached to thought.
What we're ultimately after, and implicitly describing, is the relaxation, the lack of attachment, the letting go that can take place within that kind of awareness. Letting go of grasping and resisting, letting things simply be as they are, without fighting or arguing with reality. But first and foremost you have to be able to see and feel those things, and for that you need that awareness of awareness.
"This is the way it is. You detach. You let go. Whenever there is any feeling of clinging, we detach from it, because we know that that very feeling is just as it is. It didn't come along especially to annoy us. We might think that it did, but in truth it just is that way. If we start to think and consider it further, that, too [thinking and considering], is just as it is. If we let go, then form is merely form, sound is merely sound, odour is merely odour, taste is merely taste, touch is merely touch and the heart is merely the heart."
"Let things be just as they are! Let form be just form, let sound be just sound, let thought be just thought."