Too bad you need a backrest, Outdoorsmans has their little stool that is lightweight
Closeout Item, All Sales Final The BTR stool is the ultimate lightweight seat for your outdoor adventures. Whether it’s cooking at your campsite, enjoying the scenery at a viewpoint, or sitting around a campfire with friends. The BTR is always Better Than a Rock! Features: PhantomLock™...
outdoorsmans.com
I have a couple of those little REI stools that Rodkazempour mentioned. They are cheap, fast, light and comfortable enough for a short time.
Years ago, my family had a couple of chairs like Cahunter805 mentioned, but with a flip up back. I don’t remember them being particularly comfortable.