I've been increasingly switching between biking, kayaking, hiking and running for exercise and I've come to love a very simple, incredibly lightweight extension to my comfort zone, which has other uses as well: arm gaiters.
Long used by bicyclists and runners, arm gaiters are fantastically flexible for backpackers as well. A sleeveless base layer and a pair of arm gaiters weighs about the same as a long-sleeved base layer of the same material (usually, don't quote me on ounces/grams here but mine weigh 1.5 oz. for a pair)...but has the rather massive advantage of not requiring pack removal to remove the gaiters.
My cold-night-hot-day mountain clothing system now revolves around sleeveless LW capilene tees, one pair of arm gaiters, a windshirt, an insulator, and a rain shell. I find that arm gaiters can be slipped on and off easier, and more frequently, than a wind shirt and keeps my other layers in the pack a lot more frequently. That keeps me hiking comfier, farther, and faster.
Arm gaiters to be useful when knotted together and soaked, to keep an injured or heat-exhausted colleague cool (personal experience forced this particular usage). They can be used to insulate the hand from hot objects (careful, though, typical polyester melting rules apply!) They can even be used as insulators on thin water bottles!