My Nikon D1x is fabulous, but pricey and heavy. Love the beast. It can do far more with photographs than that of which I am capable.
For a smaller camera that fits in a tank bag I use the now-discontinued Nikon Coolpix 990, which does a great job once I learned to use rechargeable nickel-metal hydride battries in it.
Today, however, there are dozens of fine digital cameras on the market from several good manufacturers, including Nikon, Canon, and others. The micro-cameras from Casio even look interesting. Don't go less than 3 megapixels, better yet more than 4.