Although I have an actual office, more often than not, my office is where ever I happen to be. With a hectic schedule, I often end up working in my car which isn't as bad as it sounds. It's better than Starbucks as I can't concentrate on my work (mostly a lot of reading and writing) with all of the people chattering and music playing. Here's the 4-1-1 on making your car into a relatively comfortable office...
First, it helps to have a comfortable car. I'm sure I wouldn't be near as content in say, a Miata, if I needed to do some serious work because I like to have everything spread out. Note: I don't do this when I am driving--I am either parked somewhere or have a driver and work as I am traveling to or from somewhere.
My office basics are always with me. I have my laptop, pens, legal pads, etc. in my briefcase. My "phone line" is my cell phone, my laptop is powered with a DC to AC car power converter (pictured above), and for internet access I have an air card for my laptop (OK but deathly slow compared to cable internet or a T1 line). If I'm lucky, I will be parked in an area that has wireless internet access (ie: some cities have this, some libraries and schools have wireless internet that reaches to their parking lots or nearby streets, also, many neighborhoods have unsecured signals).
I always have plenty of food and bottled water in my car, and, for most of the year, temperature is not a problem as the area that I live and do most of my work in has relatively moderate weather. This may not work as well in Las Vegas or Nome, for example.
Finally, pay attention to your safety. Choose a safe place to park and do not become so engrossed in your work that you forget to pay attention to what is happening outside of your car.