Without the Sun there would be no photosynthesis, no green plants no oxygen and no humans...

http://photoscience.la.asu.edu/photosyn/Default.html
Arizona State University Photosynthesis Center: information on various research projects and virtual experiments.

http://www.ktca.org/newtons/9/phytosy.html
From PBS's "Newton's Apple": easy to understand explanation of photosynthesis and activities for elementary or middle school.

http://www.sciencemadesimple.com/leaves.html
Clear description of photosynthesis, why leaves change color in fall, and activities.