If you could only take one lens

I'm considering taking a G7 for back-up. It's not that heavy. The 40mm isn't a bad suggestion. I've been thinking about getting one for video anyway, but I would still take the 24-105.

I also got a Cotton Carrier to keep my camera from swinging around which I think is going to be great for trekking.
http://buy.cottoncarrier.com/cotton-carrier-camera-vest-p/635rtl-s.htm