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- Proberly some kind of case/sleeve/bag.

- Then a external HDD is definitely recommendable to do some backups and store the files you don't want to have floating around on your computer.

- An external DVD-drive, if you'r that kind of person. Maybe if you watch some movies (DVD) or want to burn some music or video.

Other then that, you'r pretty much good to go. 🙂
 
Along with the case, backpack and usb 3.0 external hdd, I also picked up the DVI-to-TB adapter for one of my external monitors and I also got the ethernet adapter just incase the wifi is subpar.
 
I already have a bag, so I picked up a 1.5TB Seagate GoFlex drive and the Thunderbolt adapter to use for additional storage. I also picked up the external SuperDrive (because I have some software that has to be installed from a disc - only updates are available online), and ordered the Thunderbolt - Firewire 800 adapter once it was available.
 
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