Is there any way to develop software except sitting upright at a desk?

In addition to what others have said: There's more to software than typing out the code. There's figuring out what it should do, and how to implement that. That's the hard work, and you can do it anywhere.

And sometimes I think better about those things when I'm not in the office. I've done design/analysis work with a pad of paper while sitting on a rock, or with just my brain while walking.

Well, you can work at starbucks, in a bean bag, on an exercise ball or with a standing desk. People have set up stations to work whilst lying down but I've heard that's actually not good for you.

I work great laying down, either on a couch, comfy chair, or even bed. I find it's most comfy in a slouching position so the laptop has the right angle and you don't have to hunch forward haha. (Of course don't do it too much and keep active etc etc)

Standing at a standing desk also works great for me. I am also really productive in trains for some reason, even though the position is less then ideal.

Outside areas can also be great if you can make sure you can see your screen and are out of the direct sun.

I work lying down - easier on the lower back than sitting. Prop my head up with a couple of pillows, use a half-round, fairly firm pillow on my lap, rest the laptop keyboard on that at a 45. I used to use one of those mechanical contraptions with the 3 adjustable legs on each side. That works, but the laptop can also fall out of it: I had to add heavy-duty Velcro strips to the laptop and stand. No problems like that with the half-round pillow.

I use a standing desk.

I have been using a standing desk for years and I no longer have lower back pain or stiffness in my lower legs that I experienced after years of sitting at a desk.

Works for Me. (TM)

"If it works, don't fix it."

Sitting at a desk works because you can have any materials that help you in your programming right at your fingertips, whether that be a pen and paper for making quick notes or diagrams, or a calculator on-screen, or a web-page open at StackOverflow, or a man-page, etc, etc.

Planning and Thinking on the other hand, can be performed anywhere: at 3AM in bed, in the toilet, in a shower, while driving even.

For about 3 decades I've worked at a computer while sitting on the floor on cushions. I used to have the keyboard and monitor on a low coffee table, but for the last 10 years, keyboard/mouse/mac mini are on a square sofa cushion, monitor on a speaker. I don't know, I just prefer being low down, it feels grounded. Not sure why people feel they have to sit at a desk.

Not necessarily. Check out Ergoquest chair http://www.ergoquest.com/

I do it standing in front of a drawer using an old mechanical keyboard from 1988 (that has Futaba MA-41 switches equipped with O-ring dampeners) and Helix editor.