Having worked 99% remotely now for 9 years I believe I've got my mobile office nearly optimized. 

Here is the list of exactly what's need - no less and no more:

• Noise canceling headphones - At a coffee shop there is only a 5% chance that they're playing music you find tolerable and 2% it's enjoyable. It's also likely to be either way too loud or the only seat is right under a speaker. The head phones must have a hard case to protect them in your bag. These are mid-to-low end JVC ones I've found suitable for four years: http://amzn.to/A96o00  Pro-tip: Carry a couple spare AAA batteries too or whatever.
• Sigg water bottle - You need to stay hydrated and avoid plastic. Even more important is something that will never ever ever leak. An aluminum 1L Sigg is perfect.
• Spare power cord - This should live in your backpack otherwise you'll get to the coffee shop and realize you forgot it
• Power outlet expander - There is a 30% chance the outlets will be taken, 90% if you're at an airport. Something like this is ideal. Small, light, cheap, but invaluable. I get complimented every single time I pull this out when outlets are at a premium: http://amzn.to/A5ivjE
• Suitably powerful laptop - There is pretty much no reason to have a desktop at this point as your work computer. Yet I'm shocked on a daily basis at how much time people waste waiting for the laptops to boot, shutdown and spin around. A good machine pays for itself in a few months of more productive work if not weeks. Pony up!
• A baseball type cap - As the sun moves across the sky you'll eventually have light at the top of your vision or a car will park with it's windshield focusing the light laserlike at your retinata. A baseball cap can mitigate this annoyance. 

That's everything you need for the perfect mobile office. Happy mobile computing!