Two mornings each week, I volunteer to teach physics to local 12 & 13 year old kids.
They're homeschooled kids; we meet at one of their homes for 4 hours a week. I'm teaching the science class that I wish I'd received when I was in 8th grade.
3 months of Newtonian physics, then a month on wave mechanics (made a glass wave tank!), we're now finishing thermodynamics and will soon start E&M. Heavy on experiments: bicycle wheel gyroscope, conservation of momentum when throwing a football while standing on a skateboard, entropy & heat of crystallization using Sodium Acetate. We use the physics apparatus that I've collected over the years ... some professional equipment and a lot of homebrew demos. An oscilloscope that cost $25 at a yardsale.
This past Tuesday, we measured the distance to the sun by comparing the warmth of sunlight on the kids hands to the warmth from a 100 watt incandescent lamp. By adjusting the distance from hand to lamp, they found the distance from the light where it was "just about as warm as sunlight". Then they looked up the solar luminosity and used the inverse square law to deteremine the astronomical unit. Got it to with 30 percent of the canonical value. (of course, Slashdot people will see the circular reasoning here, but letting the kids figure that out is part of the fun).
No tests - it's immediately apparent when someone doesn't get something, and when to take a different approach. Occasional homework (always an experiment: for instance, determine the vertical distance (in meters) from the sidewalk outside your house to your bed. Knowing your mass in Kg and the gravitational constant, find the amount of work it takes to walk into your house and go to bed. Notice that there's no "right" answer to this question, and it's unlikely that two kids will get the same answer)
Parents often bring muffins & goodies; the kids are curious, enthusiastic, and motivated. Best part: I take home a broad smile ... it's the high point of my week.
A friend of mine - a PhD chemical engineer - volunteers at the San Francisco Exploratorium. Another friend works as a docent at a nearby bird sanctuary.
All of us are busy, yet each of has something to contribute. Mix your interests with enthusiasm, toss in some creativity, then get out there and volunteer. You'll never know how much fun it'll be!
-Cliff on a sunny Saturday morning in Oakland, California