This online lab utilizes a flash animation (http://www.kscience.co.uk/animations/photolab.swf), which simulates an Elodea plant and tracking the number of oxygen bubbles it produces to determine the rate of photosynthesis. I use this in my General Biology class.
In the simulation, you can change if carbon dioxide is added or not, the light intensity, light filter color, and temperature of the water. I have students compare carbon dioxide added/not added and temperature. They have to create hypotheses, answer questions about the simulation, determine variables/controls/constants, and graph data.
I also provide an extra credit opportunity in which they design a study of their own in regards to the color of the light (which you could easily edit and make it assigned, if you want).
The lab is pretty simple, but you could easily add more/make it more detailed if it suits you. It would work just as easily for middle school students as it would for Gen Bio students. I would definitely add more to it if I were planning to use it with my AP students.
Most of my students easily finish in our 45-minute time period. I assign anything that they don't finish for homework.