When we recently took a family cruise, I made sure to pack my backpack with all the essentials needed to create a home office from the road.
Along side my HP tablet PC were my broadband wireless aircard (essentially useless hundreds of miles out to sea), my Ativa Memory Stick card reader, a USB flash drive, and a Monster five-plug, surge protected power strip. You never know when you’ll have to plug the laptop, camera charger and some other device into one single outlet.
I also carry a stretch of ethernet cable, and a clutch of adapters.
Here’s what some other people had in their backpacks and laptop bags. Wireless Road Warrior found size essential. Webworker Daily had a review of the stash in one laptop case (read the comments for some more good ideas). Here’s some more threads on the topic.