A closed-cell foam sit pad works for me. Cut from an old sleeping pad, or you can buy them. Also goes under my feet at night. I've sometimes carried a closed-cell pad for sleeping, and while not the most comfortable for that, it makes a great chair for lunch breaks and general loafing. The 3/4 size is long enough that you can sit on one end and use the rest of the pad for a back rest, propped up against a rock or whatever's handy.
I currently use the same thing, a small piece of my 40 yr old ensolite pad. I also put the piece under my feet at night. It provides insulation, padding and also saves the seat of ones pants from abrasion, especially when sitting on rocks! It weighs probably about 5oz.
Used to use an old, aluminum beach chair. Switched to an REI Flexlite when that got worn out. Loved it except for the tendency of feet to sink into the ground.
I do miss the ability to lean back and really relax, but once in a while you can find a "friendly rock" that lets you lean back.