Here is another idea from my favourite art book for toddlers: First Art by MaryAnn Kohl.
This is a favourite here. Dip golf balls in paint and roll them around in a tray lined with paper!
Our tray was a bit too large and not so easy to tilt by Finn, but he still enjoyed it!
And so did I. I continued long after he had lost interest. Erm.
What are your children's favourite painting activities? Which ones do they ask for again and again?