Color mixing is a fun way to learn about primary and secondary colors. Just squirt two primary colors (red, blue, yellow) onto a paper plate. Kids can mix the colors together using their fingers, a brush, a sponge - anything.
No activity happens without N begging to use cars, so we took it a step further and made a car painting. N drove his cars and trains through the newly-mixed green paint, then drove them across paper to make tracks. He had fun experimenting with different cars and trains and seeing the fun designs they made. If you're looking for a quick, simple art project, give this one a try!