As I wrote last week, I don't really need an office anymore. Here's a check list of what keeps me mobile:
iPhone, though I used to use a BlackBerry and have used an Android phone also. The important thing is to have constant access to email, voice and SMS or instant messaging regardless of geography. The iPhone scores over the BlackBerry by being great at web browsing and viewing of Excel sheets and PDF attachments. I can't travel without a charge, though, and top up the battery with power as often as I can throughout the day.
iPad which gets me through the day on a full charge, and is so very much more portable than a laptop. The WiFi-only version is fine because I use the "Personal Hotspot" feature of the iPhone to connect to the Internet while out and about. This works great apart from the iMessage facility that currently doesn't know to check for an Internet connection over Bluetooth so I have to stick to using iMessage on the phone when the iPad is not on WiFi.
Also in my bag are headphones; a charge / sync cable and a micro-USB charging cable for my Jawbone Bluetooth headset. Oh, and a pen. I almost never need that (Autosketch SketchBook Express gives me all the virtual paper I need).
The final thing in my bag is an iPad VGA adapter: along with SketchBook and a projector I then have a replacement for flip chart or whiteboard.
This lot fits easily with room to spare in my STM bag and means I can run for a train and spend a day working and travelling without breaking a sweat.