Learning the Montessori way is really fun!
We are starting on subtractions and the kids are doing really good. One way that helped us achieve that is this fun activity with paper plates and rocks. I used space rocks because of our Space themed science unit but you can use really anything that you normally use as a counting prop (buttons, beads, glass rocks, M&Ms, etc)
It's easy - you just write the subtraction problem on a paper plate using sharpie and give the kids some rocks. To help them figure this out, I told them to start with the higher number and put that many rocks on a plate. Minus means to take away, therefor they would have to take away from the higher number the smaller number. How many rocks are left?