I'm doing something new with our b'nei mitzvah kids this year. (Credit where it's due: this is an idea I adapted from my friend and teacher Rabbi Burt Jacobson of Kehilla Community Synagogue in the Bay Area.) It's called Mitzvah Experimentation.
I brought this to our seventh graders in our first Hebrew school class of the year. The first thing we talked about was, what's a mitzvah. Some of them said "good deed," which is a fine answer, though not a direct translation. Others said "a commandment," which is what the word mitzvah means. A mitzvah is something which we are commanded to do, or to not do.
Commanded by whom? The most traditional answer is God. That word raises some eyebrows. Not all of my students are certain that they believe in God. What if you don't believe in God -- does that scotch the mitzvot?
There's a story about Reb Zalman z"l, the teacher of my teachers, faced with someone who didn't believe in God. He asked that person to tell him about the God they didn't believe in. Because "maybe the God you don't believe in, I don't believe in either!" Over the millennia we've thought about God, talked about God, and described God in all kinds of different ways. Some of those ways work for me. Some don't. Some might work for you; some might not. The name "God" can mean a lot of different things. And if my students want to talk about that, I'm happy to do so.
But when I go deeper into the question, what I hear is: if I don't believe in God, do the mitzvot matter?