Here is an idea that combines Science, Solar hot water and Computers......
The company Dataq offers a low cost ($25) data acquisition unit (DAQ) to students for free.
A DAQ attaches to your PC and reads an electrical voltage from a sensor and records the data to a computer file. You can make some very simple sensors that can respond to light or temperature for about a buck apiece. ( a thermistor in series with a resistor make a temperature sensitive voltage divider. The output of which can be logged)
A simple experiment with temperature sensors is to compare temperature gain in various types of solar collectors. You make a couple of boxes and paint the insides black, white or lined with aluminum. THen you place a temp sensor inside the box and out side the box. You can also add a light sensor to record light intensity.
Then place the boxes out side under some shade or an umbrella and start the data logger. wait a few minutes to record some base line data then remove the shade and let the sun shine directly onto the boxes and record the data for a few hours or all day. Then use the data you collected in excel to make some charts and plot temperatures for the different boxes along with the light intensity. You can repeat the experiment using different types of glass or plastic to see how this effect the temp rise in the boxes.
You can do a science project on just how to measure temperature with a thermistor
http://en.wikipedia.org/wiki/Thermistor
http://www.me.umn.edu/courses/me2011/labview/LabVIEW-thermistor.pdf
http://www.doctronics.co.uk/voltage.htm