For use as a weapon, try a Zenit. Only camera to have been made out of single die-cast piece of metal. As used by the Soviet military (it's still a little crap).

Yashica Electro 35 is very inconspicuous and has a good strong metal chassis. The 45mm f1.7 lens is a stonker, bright and sharp (speaking from personal experience). I got my Electro for 30 pounds of eBay and it works like a dream.

The camera that really packs a punch without getting noticed at all is the Minox 35 series. Smallest 35mm full-frame cameras ever made, really tiny and the Makrolon construction makes it look really cheap. A friend of mine once though that it was a disposable camera! To the contrary, the Minox 35 had an astonishingly sharp Tessar-type Minotar lens whose results continue to astound me.

For the ultimate bang for the buck, Olympus Trip 35 available for $10. Very sharp lens for such a cheap camera.

All these available from the usual stockist (eBay).