Instructing the Ignorant
The truth of the faith is only very rarely simply endowed on men and women by God. The normal method for obtaining to the faith is through instruction. Given the breadth and depth of the faith we live as Catholics, learning about the faith is a lifelong process. Children, adolescences, teens, young adults, adults, and even the elderly need instruction in the truths of the faith. Once we understand and know things about our faith it is incumbent upon us to help others who may need instruction. This does not mean that you need to run out and become a Sunday school teacher, although that work does fit very neatly into this spiritual work of mercy. No, it often times means simply instructing your children, talking about the faith with your wife, answering your relative's questions, and on the whole being informed about your faith.
Part of this of course means that you yourself must be diligent in the lifelong learning process. Keep digging into the truths of the church and keep learning about it. No matter how deep you go, I guarantee you, there will still be more hidden gems awaiting discovery. Some common examples of volunteer opportunities for this spiritual work of mercy are listed below.
Childhood Faith Formation
Your local parish almost certainly has a childhood faith formation program. Maybe you call it Sunday School. Maybe you call it CCD. Whatever you call it there is a program at your parish that is working to pass on the truth's of the faith to the children of the parish. They always need help, maybe with the little children, maybe with the teenagers and young adults, but there is always a need. The need might be as simple as having a second person who is Safe Environment qualified to be in the classroom and who can assist the teacher. The need might be for a catechist. Whatever the need is, please consider helping, they always need help.
Adult Faith Formation
Hopefully your parish also has adult faith formation. It might be in the form of Bible studies, or of Formed video discussions, or other activities, but your parish should have a program or programs aimed at adults to help them deepen their faith. If they don't, inquire with the pastor or parish council about starting one up. If they do, see if they need help, or if there is an area of education that they are lacking that you might be able to provide.
RCIA and RCIC
Your parish should have a program called The Rites of Christian Initiation for Adults and for Children. This program is for people who are outside the Church and who want to come into the Church. Like the Faith Formation programs they almost always need help. For the adults there is always a need for catechists, people to teach specific topics to the people coming into the church. For the children there is always a need for Safe Environment qualified teachers and helpers. Inquire with your parish RCIA director about what help they might need.