I finally convinced my children that you didn't always need to use brushes when we have paint. I had my kids pick 2 colors of paint and then we put a puddle of each color inside the glass ornament and they had to tilt the ornament all over in order to have the paint cover the entire inside. My son decided that his looked like the earth. :) They all look really cool.
paint (I used tempera)
clear glass ornaments (I got mine from Michael's)