One of my very favorite pieces of gear is my 'sit pad', which is just a piece I cut off an old RidgeRest closed-cell foam pad. It is approx 12" x 13".

When I am hiking it is in my pack and I wrap it around my 2 liter Platypus bottle to keep the water inside it a bit insulated from warming up as I walk.

When I am awake and not hiking, I use it to sit on and to cushion my tailbone from the hard ground. Often I lean against my pack, which is in turn leaning against a tree or a rock. This takes care of my butt and my back as I sit.

When I am sleeping, I slip it under my sleeping pad so it can provide a bit of extra cushion under my hipbone, because I am a side-sleeper and my hips like the tiny bit of extra cushioning. I sleep better this way.

It weighs a bit more than an ounce (36 grams).