This is a great practical Q, in this answerer's opinion.
Serving the Lord is as subtle as continuing to breathe in and out, to show up with a smile, to encourage with a word or a gentle touch, to just 'be present', keep company, hold the good thought, extend love - whether through thoughts or small actions.
The elderly man who can't get out, but who loves God, may serve by touching the lives of those who are around him by his patient example of acceptance with grace, and his life's wisdom shared when needed with those who help him.
The disabled woman, who loves God, who cannot see and relies on others to get her places and help her, serves by showing up daily with wise counsel and a cheerful loving appreciative heart, who gives the good example of living within restricted circumstances does not restrict creativity, imagination, or clear compassionate communication, who can show that there is always some way to help in small physical ways when she doesn't allow an adversarial situation to overwhelm her.
The mother, who loves God, who has children who take up every minute of her time during the day, contributes and serves by showing and speaking from the only place of strength - a strong personal GodRelationship, as she demonstrates the example right in front of the children, by extending patient love and forgiveness that is endless, by listening constantly, by feeding clothing and nurturing, by giving up her 'self' for the good of 'the children' without complaint.
Firemen, policemen, teachers, nurses, day care providers, cooks, farmers, busboys, diswashers, stable cleaners, sewer workers, mechanics, etc, even the homeless, who love God, all serve by extending love and holding the space of compassionate forgiveness. No act of kindness is ever too small to be huge in the heart of God. That which we do to the least of our brothers we do towards God.
So, keep 'suiting up & showing up' no matter how ya 'feel' and give a smile that lights a heart, an understanding ear that , or lend a helping hand, and that what helps someone's broken heart heal.
Thanks for the Q :)