This is a very simple activity, but Jonathan really enjoyed it:
We recently read White Rabbit's Color Book in which the white rabbit had fun bathing in different paint colors and seeing which color(s) she ended up. Jonathan loved this book, so I thought it would be fun to mix primary colors.
I pulled out three small glasses and filled them with water. I also pulled out a washcloth to mop up any spills. Then I added food coloring to the three glasses to make them turn red, yellow, and blue.
I gave Jonathan a medicine dropper and he had fun transferring the colored water back and forth. He was amazed when he combined blue and yellow and made green water.
This literally kept him occupied for an hour. I would periodically refresh his water and he just had tons of fun!