Photosynthesis, science fair
I am assuming you are in grade school.
The problem is, as was mentioned before, that photosynthesis is very complicated.
The formula you know of is extremely simplistic and skips over 100 reactions on the way. you can find plenty of information in Wikipedia, or if that is too much you can use the Simple Wikipedia.

In high school we took some kind of water plant or algae and put it in an upside down test tube filled with water, as oxygen was formed it filled the end of the test tube. This is a fairly simple way to monitor photosynthesis and you can use it to study photosynthesis. for example: how does the temperature effect photosynthesis, how does the color of light affect it etc.