We first heard about "tall painting" from Teacher Tom's blog here. Go ahead and check it out, I'll wait.
I've been wanting to try this for some time, and when we picked up some Halloween pumpkins the other day, I decided they'd be a great medium for the process. My daughter (3) wasn't interested in waiting for me to fill cups with acrylic paint (I chose acrylic over tempera as it's thicker), and she dove right in, squirting the paint right on top of the pumpkin, one color at a time. We used some old craft paint I had on hand, and she was welcome to use as much as she liked, because it was becoming too gummy for me to use.
When she was done painting, she applied a liberal coat of blue and green glitter.
Didn't it turn out beautifully? It took about a day and a half to dry completely, and it sure does brighten up our front porch!