Sun Prints are very easy (if the weather cooperates)
Any of their favorite toys will work well too! Even other construction paper cut outs!
Note: The papers need to be in direct sunlight that is constant. If the sun is fading in and out this won't work. (That's what happened to ours today) :(
The first picture was retrieved from google by searching "construction paper sun prints" since ours didn't work today. I hope you have better luck then we did!!!
You can also buy sun print kits on Amazon.com for anywhere between $6 and $20. These come with chemically treated paper that reacts with the sunlight. These work very well and aren't quite finicky as the construction paper is.