Food and water.
I can live quite happily without a camera.
BUT, the bare basic kit would be at least one body with lenses to take me from 12mm up to 300. Sigma 12-24, Nikon 18-70 kit lens and Nikon 70-300VR. If I'm allowed a bit more space, lob in another body (35mm, F80) which'll let me use the full wide angle capability of that 12-24, giving me the field of view (35mm equivalent) of 12mm-450mm. Since there's still space in the bag, I'll take an 8mm fisheye as well - then there's also space for a 50mm f/1.8 in case I need the low light capabilities. If I reckon I'll need some support, I'll take it - either a Pod in a pocket or either a GorillaPod SLR Zoom or a "proper" tripod strapped to the backpack.
As for gadgets and gizmos... I've always got a blower brush in the bag as well as a lenspen and cleaning tissues/fluid. A torch is handy so there's always one of them in there too - along with a spare set of batteries for it. While we're talking batteries, there's always a spare one for the DSLR in the bag and a set for the F80. There's certain to be a few GB of cards in the bag too. A couple of Circular pols complete the kit pretty much - apart from some extra film. Since it takes up almost no space and is very light (not to mention that it stops all the water flowing off the sides of any seascapes...) I also have one of those little digital levels to help me keep my horizons level.