I don’t know if this is officially a Montessori activity or not, but it seems like it could be. We cleaned several non-shiny pennies using old toothbrushes dipped in a mixture of vinegar and salt. This exercise was great for focus and concentration for both my girls. The 20 month old focused on being careful and not spilling, while the 3 1/2 year old was concentrating on getting the coins as shiny as possible and the science of watching the tarnish dissolve. It kept the older one focused for over a half hour. It also helped the younger one especially with her hand eye coordination.
For more great Montessori ideas, check out the links and posts at Mommy Life.