I used to take a collapsible three-legged stool with me when I went out urban sketching in watercolor or plein air painting. I bought it IKEA in Hong Kong years ago. When I first got it I thought it was fantastic! I had a softer place to sit rather than on walls or sidewalks. Then when I traveled by air to other places I wanted to take it with me but it added more bulk and weight to my suitcase.
When my fellow WAM:Women Artists Mentors and I were planning our trip to Venice last spring at one of our Google Hang Out meetings I asked the others if they were taking stools for sketching. I showed Helen Beacham my stool and said I always have this dilemma packing it number one, then lugging it around on a day's outing, number two. Helen immediately replied, "You need one of these!," and showed me the Coleman Camp Stool. I ordered it off of Amazon then and there. It fits easily in my plein air bag, it's not too heavy where you aren't super uncomfortable carrying it around all day on outings while you are traveling, and I find it to be fairly comfortable for what it is! It even comes with a small carrying case! Thank you Helen, I could kiss you! I use it ALL the time!
I'm going to put the link here in case you are as anxious to get one as I was!