To me, sadhana is a daily spiritual practice allowing time and space for an individual to turn inward.
As Yogi Bhajan (of the Kundalini yoga tradition) says, "Sadhana is self-enrichment. It is not something which is done to please somebody or to gain something. Sadhana is a personal process in which you bring out your best."
Sadhana could be taking a walk in nature, doing breath work or yoga asanas on a mat, spending time meditating or chanting, reading and reflecting on a poem, or simply watching the sun rise.
Please accept this post as a possible starting point for your own practice today.
From In the Body of the World, by Eve Ensler (found via Brain Pickings):
I was raised in America. All value lies in the future, in the dream, in production. There is no present tense. There is no value in what is, only in what might be made or exploited from what already exists. Of course the same was true for me. I had no inherent value. Without work or effort, without making myself into something significant, without proving my worth, I had no right or reason to be here. Life itself was inconsequential unless it led to something. Unless the tree would be wood, would be house, would be table, what value was there to tree? So to actually lie in my hospital bed and see tree, enter the tree, to find the green life inherent in tree, this was the awakening. Each morning I opened my eyes. I could not wait to focus on tree. I would let the tree take me. Each day it was different, based on the light or wind or rain. The tree was a tonic and a cure, a guru and a teaching.