By light sensor I guess you mean like LDRs?- Light Dependent Resistor? They're fun.... I stuck one on the end of a servo horn and had the servo "hunt" for light....
I'd get a few small transistors too, BC547 style. You can then pretend the LEDs can't be run directly from a digital pin and switch them on and off with a transistor as if they were "real" plant as shown here.
Maybe also a couple of cheap toy DC motors, run them with the transistor, remembering to read up on the need for a diode...
Have a search across various Arduino supplier sites- they tend to have pre-packed sets of odds and ends.