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: a passage from The Unlikely Pilgrimage of Harold Fry, by Rachel Joyce:
He had learned that it was the smallness of people that filled him with wonder and tenderness, and the loneliness of that too. The world was made up of people putting one foot in front of the other; and a life might appear ordinary simply because the person living it had been doing so for a long time. Harold could no longer pass a stranger without acknowledging the truth that everyone was the same, and also unique; and that this was the dilemma of being human.