Little Bear is terribly young for many of the (out of the bathtub) water and sand activities I have seen suggested by Montessori curriculum.
However, this Montessori mom had a wonderful idea for babies who have little motor control (those who make a mess) and still put anything and everything in their mouths:
Use puffed cereal (aka Rice Krispies) as a way to teach "transferring" - pouring an item from one thing to another (such as a spoon to a cup, or a cup to a bowl, etc.)
I loved this and so did Little Bear. Fortunately, we have a dog who enjoyed eating the puffed cereal that fell on the floor.