Dirty pennies + White vinegar + Baking Soda =
Happy giggly, thoroughly entertained boys and clean pennies.
I let the boys play in the kitchen sink and they were in fits of giggles over the "explosive" baking soda and they enjoyed scrubbing down the pennies to make them shine. I had thought of letting them use old toothbrushes to scrub the pennies, but I know Wesley would have stuck the toothbrush in his mouth, so I just gave them dish cloths to scrub with.
It would have been a fairly hands off activity for myself except for the fact that I enjoyed watching their response to the baking soda and vinegar so I spent the entire time watching them play instead of working on other things in the kitchen.