The most useful item in our backpack might be something we originally thought was a complete waste of time. P was visiting the wine regions of Spain, and in the Rioja Alavesa his hosts gave him a very simple little black nylon bag that wasn't much bigger than a single sheet of paper. The drawstrings on the bag not only closed it up, but they attached to the corners and were long enough to serve as shoulder straps if you wanted to carry the thing like a backpack. It doesn't weigh three ounces.
Which is the only reason that he threw it into his suitcase and hauled it home. In fact, somehow he ended up with two of these.
Because it was so light, P used one as a stuff bag for his down jacket for a while...and it worked great. The combination also made a pretty nice pillow to sleep on, especially when wrapped in a soft fleece jacket. So it became part of our gear for most trips.
And then on one trip, he realized that it was the prefect size for him to use to pump water. You know the exercise: take four or five bottles of water and the pump down to the river or lake....pump the water into all the bottles...and then figure out how to carry and juggle all those bottles back to camp. Well, it all fits into the little black nylon bag perfectly.
In Emigrant Wilderness, we used it as a day-pack to carry our lunch and water for a day hike to Kole Lake and beyond...which is what it was originally intended for, and it worked just fine!
When our pump got clogged on a (muddy) spring trip to Carson-Iceberg Wilderness, we used this little black bag to help us melt snow for water, laying it in the sun with a pot full of snow inside. The solar heat in the black bag worked like magic.
And on a last trip to Mono Creek, we were pumping water from the creek while standing in snow. That water was ice cold, and M left a few of the water bottles in this little black bag afterwards. When the sun hit them, we quickly discovered that our little black bag had turned our clear plastic water bottles into a very effective and nicely warm sun shower. We used the warm water to wash off a bit...and also to reduce the amount of gas we needed to heat our dinner.
And the bag seems to be pretty waterproof, so now P uses one inside his back, up against his back to serve as a moisture barrier between his sweaty back and the contents of his pack.
OK--we've lost track of how many uses this thing has...but we now carry both of them on every hike. And we are on the lookout for more!