I've realised that the Geometric Soilds would make excellent first blocks. What other block set has such a range of shapes? What other block set has a sphere, a cone, a cylinder?

These Geometric Solids are Caspar's and we've been using them as a sensorial material with his mystery box or bag. It wasn't until I introduced them to Otis that I discovered what great blocks they make, for building, for stacking, for feeling. 

These are now on my must have list of Montessori materials. Otis can use them as a toddler at the same time Caspar is using them as a five year old. Otis is feeling them, Caspar is naming them. Sensorial, mathematical and language. Brilliant!