Normally, I do not use a chair but rather an insulating foam pad, like this one:
35 g, hi.
But for ultra-light camping and summer activations that will be a bit longer, I sometimes take this foldable chair:
It is only 250g and really small, yet a big improvement over sitting on the ground. I also often carry this with me for train rides in busy seasons, for it will turn a ride in an overcrowded ICE train into a much more acceptable experience as compared to standing or sitting on the floor.