Renewable energy systems
That budget sounds tight even for a toy-sized demonstration of such a system, especially if it must be functional. The energy levels involved will be miniscule, and the losses throughout the system will kill you unless you use expensive precision components.

It sounds like this is a science fair project, and as such, it might be more effective if you were to research such systems and demonstrate your knowledge by producing a software simulation of how they work, using real-world numbers. It could have nice animated graphics, etc., that show the flow of energy throughout the system under various operating conditions that a user can "dial in" &mdash; things like the amount of sunlight, wind speed, rainfall, etc.