You can use a led as a proximity switch, pulse the led on and then look for a light reflection.
The capacitance of a led is changed by ambient light, what I'm getting at is that you can measure ambient with a led by measuring its capacitance, I cant remmeber if you forward or reverse bias the led, but if you charge its capacitance via a high value resistor you'll be able to measure the charge time, and therfore the capacitance, ie ambient light.
If you google it you'll find some applications, adapting one shouldnt be difficult.
If you have a capacitance bridge have a play around.