This is another common idea from pinterest (see Teach, Play, Love- my boys here) adapted to what I had available.
I used large ziplock freezer bags and paint (a lot use coloured hair gel but I don't have any hair gel!). I wanted C to learn to how colours mix to make other colours so I used 2 colours in each bag, and sellotaped the top shut, after expelling any excess air.
C got a lot more from this activity than I expected him to. First of all was the fun of 'squishing', and the fun of 'spreading out' the paint to fill the bag. He used his hand and cars for that.
Then he 'drew' in the paint with an empty pen shell on the table and then we stuck it on a sunny window pain and had a long session mark making with the empty pen, his fingers and even the sellotape roll. The resulting 'rainbow' looks so lovely on a sunny Spring day!
It was a great success and I recommend it to anyone.