Usually, Montessori schools present and study artists, Cezanne, DaVinci, Degas, etc. You can take these artists one at a time, show examples of their work, talk about their lives, how things were when they were children. Most children are fascinated by all of this. This also works well for classical composers. Then the children can try and copy the pictures, or make their own pictures using the style that the artist used. You can really tie art, history, culture, together this way.
Other ideas, more along the lines of crafts:
You could make play-doh. This could show color blending, plus incorporate measuring when making the play-doh.
You could do nature prints. Take a walk and collect things outside and then let the kids dip them in paint and make prints of them.
You can play a piece of music and have the children "draw" it
Draw with crayon and then paint watercolors over it. This demonstrates how wax repels water.
Paint on half a piece of paper, then fold in half. A good lesson on symetry.
Clay is always a wonderful 3-d medium at that age, something that will dry overnight and can then be painted.
Outline each child on a large piece of paper and let them color themselves.
That's all I have for now, if I think of more, I will post again.