Get a Thermarest. Folds up fairly small, only 1.5lb, super comfortable and pretty warm.

Pair this with a waterproof bivy sack, my favorite is the military surplus Goretex bags like this. I've seen people stand in ponds up to their chest with one of these on and come out dry.

With a 0 degree bag you're ready for just about anything and will be mostly comfortable in just about any weather.