I've seen this linked at a couple of sites now. It's hypnotic, and a bit disturbing. Neverending Fall is a flash animation of a woman freefalling through a field of randomly generated spheres. You can click on her to move her if she gets stuck, or if you just want to fling her around. A part of me is convinced that Hell is real, and this woman is there, now, and every time someone loads this program the horror begins anew. But it's so fascinating...I can vouch from personal experience that it can provide literally hours of entertainment. Showing it to K may have been a mistake.
A far less unsettling, and somewhat less engaging, flash person you can play with is the Liquid Man, who floats rather more peacefully around your screen.
Crazyimages.com is a nifty site with lots of java tricks and animations, some of which are you can play with, and some that are just cool to look at.
I may have posted this some time ago, but it fits this category so here it is again: an interactive Mona Lisa with a range of facial expressions, presented by the Cité des Sciences.
Finally, there is the stylish Fat Dollmaker, a plus-sized paper doll which could be a fun and positive role model for kids, if not for all the risqué clothing options. So it's a fun and positive role model for not-kids.