Building a waterproof and cheap proximity detector

I'm new to electronics, so I would like to directions for building an effective proximity sensor.

Requirements:

1. Must detect objects within 10-50cm.


2. Waterproof.



3. It may be exposed to sunlight.


4. Distance detection a plus.


5. Cheap. Really cheap.

Basically I'll put it on the ground, that's why it must be waterproof (rain) and may be exposed to sunlight. The object I'm trying to detect is metallic.

Are there other solutions than using IR/Ultrasonic sensors?

Thoughts?

Thanks