I have this idea in my head that'd it'd be fun for Daddy to paint me with multiple colors in multiple designs and then when we're done hose me off with the hose and all the colors will run at first befor being washed off and it'll be neat!
Unless your allergic.. try liquid latex, you can splash, or paint it on and when it sets you will really feel a second skin.. peeling it off is fun too..
Get one of those really, really large rolls of paper and have him use your body as a stamp. He can paint various parts of you, press you, clean you off, then try again. I can't see any great art coming from it, but it could be fun.