Setting up an open-ended, process-based activity like this is a great way to encourage curiosity, creativity and exploration through undirected sensory play.
What you need:
Paper
Paint and other mark making materials
(Top Tip: use non-toxic, washable mark making materials)
As a child, I loved playing with the little circular plastic ball mazes that I found in Christmas crackers and party gift bags.
I now know that playing with ball mazes develops hand-eye co-ordination and that making them is a great STEM activity promoting the development of a number of skills including designing, constructing, problem-solving, ...