The Napsack is one of the best examples of human innovation since sliced bread. It has zippers at the shoulders, so you can stick your arms out, and a cinch at the bottom, so that you can open it up and stick your legs out.
Hike it up to your waist, cinch it, and wear it like a puffy coat around the campfire, and then crawl right back into your tent without ever having to leave the warmth of your bag. Perfect for summer trips, couch surfing, music festivals, jumping into after snowboarding, surfing or any other activity that brings your core temp down. Its not too hot for inside and is awesome for wearing around the house in the winter.