A Photon with a white led and a 2032 will produce usable light (enough to find your way around pitch dark with) for 150+ hours. And having a bunch of lights with you can be a big help in a group situation.

Note: lithium AA's (made by Fuji, I believe) are available for $1.00 each (qty 20) from www.batterystation.com . They have been tested extensively at CPF and their characteristics are a bit different than the much more expensive Energizer L91 cells. See CPF for details but my interpretation is that they're not -quite- as good as the Energizers, but their advantage over alkaline is very similar. They may offer lithium AAA's sometime but right now they don't.