At Least One Pair of Earbuds
Even if your listening gear of choice is a sweet pair of Sennheiser cans, you might not carry them everywhere with you. Be ready for YouTube tutorial vids at the library, or an impromptu jam session on the train, by keeping an old and compact pair of earbuds handy.
Try this: iHip's retractable earbuds are compact and inexpensive at around $12.
A Backup Phone Battery
A long day out of the house takes as much energy out of you as it does out of your smartphone. Make sure you stay connect late into the night with a backup battery.
Try this: Mophie makes great backup gear. Try the Juice Pack case for iPhone or the Samsung Galaxy S III. Or the keychain-sized portable battery Juice Pack Reserve for iPhone or other Micro USB smartphones.
An Empty Flash Drive You Can Afford to Lose
You might keep a personal drive to travel with important docs, but you might also want to keep a cheap spare on you. If you ever need to give it away or leave it with someone, you won't panic thinking about losing your gear or your data.
Try this: A 1GB drive will cover you in a jam for less than $1.