So I facilitated a watercolour painting session with a group of people with dementia. We used impressionist works and it worked really well. See these links for some good Van Gogh:
Alternatively, a quick Google images search (selecting line drawing in the options) will find you Renoir, Monet, Manet, Cezanne…
Here’s an example of a finished product. Just to set the scene a little for you, this client has mid-late stages dementia and consequently can barely string a sensible sentence together. Displays serious sundowning behaviour at times. Yet, she worked happily on this masterpiece (personally, I think it gives Van Gogh a run for his money) for just under two hours.
Think that’s what you meant when you said ‘flow’, Mr. Csikszentmihalyi. Love it.