If you are in mostly warm weather the US Military produces something called a poncho liner, or "woobie". Trust me, infantrymen get so attached to them they steal them from the service and keep them when they get discharged. It is basically a fairly thin insulated comforter in camo that is intended to go underneath a poncho to use as a waterproof sleeping wrap. They keep you surprisingly warm. On a warmer note, you can do the same thing with a thicker quilt.
I wouldn't suggest wool blankets or the like. They work, and work well, but are heavy and bulky in comparison to some of the newer things you can get.