Maybe the lawn chairs are a good thing, because they encourage you to stand up and take a break every once in a while? A few people have standup-desks at Google, and ergonomically they seem much better than the regular sit-down ones. Humans weren't built to sit in anything for 12 hours per day.
My favorite seat is a flat, backless piano bench. Not only does it encourage movement, also does it encourage great posture. ;) It's a bit like sitting on the floor, but a wee bit more ergonomically friendly.
I had a comfy bean bag chair set up for a while, and I think it was sort of good for exercise because I would drink a lot of assorted beverages (water, iced tea, coffee), so I would have to get my ass off the near-floor quite often, and the effort was greater than if I were sitting in a normal chair.
It also made it easy to slip into a short nap if I felt like it. :)
