Jonathan talks about essential things to pack when traveling.
You'll need a bank card or credit card or something to access your money. You don't always want to carry around a lot of cash, so a bank card can be good.
You'll need at least one change of clothes. You certainly don't want to be walking around smelling bad and feeling uncomfortable. You can usually wash one pair of clothes, while you wear the other one.
And of course, you always need a pen. From the moment you arrive in a country, you'll need to fill out the immigration forms using your pen. You'll use it many times to write down the information when you need like directions, plus you can use your pen to write down the contact information of new friends you make so that you can stay in touch with them into the future..