The Lansky Dog Bone is a good ceramic field sharpener and very light. They make a triangular one that fits the Spyderco serrations.
DMT makes several folding diamond sharpeners. The
Mini Sharp is one of my favorites as it folds and has enough handle to keep your fingers out of the way.
The lightest, most compact diamond field sharpener I have found is the
EZE Lap model H, which is a flat 3/4" x 2-1/2" 600 grit stone with a hook groove. It comes with a snap shackle and split ring to use as a keychain. I just used a loop of paracord on mine and it stays in my PSK.
EZE Lap makes some credit card sized flat diamond stones down to 1200 grit.
http://www.eze-lap.com/product/creditcard.htm. That 400/1200 kit for $20 is tempting.
If I were going for broke, a small 600 grit diamond stone and a Lansky Dogbone would give great edges in the field. I carry the 600 grit EZE Lap H as a minimum in my kit, aiming at really messed up blades. You might not get a shaving sharp edge with the 600 grit, but you can get enough edge back to keep working.
You could carry a little sandpaper for touch-ups --- you won't get much lighter or cheaper than that and it can be used for other things. Also, Fallkniven makes a small whetstone that is ceramic on one side, diamond on the other. It works great, but its a little on the heavy side for hiking.