In a search for websites that are interactive for children and even infants, I came across JacksonPollock.org. Simply move the mouse on your computer and you can create a Pollock-esque work of your own:
For infants as young as six months old, the site provides a few stimuli: holding and moving the mouse (or hitting the finger pad on a laptop) and seeing the high-contrast black-on-white imagery that appears in response to their movements. For older children, the elements of composition and color (the background and "paint" colors can be changed by pressing computer keys) come into play.
All of that said, the site's a lot of fun for adults, too. But that's the proof of the best playtime occupations: their ability to engage both young and old. Check it out for yourself at JacksonPollock.org.