Buy your first transportation. Check if you need an onward ticket to enter (e.g. US). If you do not know how long you will stay in a country buy an onward ticket and cancel it. Most airlines have a 24-hour cancelation policy so you can purchase a ticket, print it, and then cancel it. Another option is to purchase a cheap bus ticket to another country.
Extend the expiration date of your passport and credit cards if needed.
There exist a few vaccinations which are really recommended in some countries. Call the tropical medicine for further information. It is expensive but normally it is just once in your life.
Save in your email, dropbox or google drive: Transportation tickets (if not on an app), Hostel reservations, Insurance information, Contact info for credit card companies, Emergency contacts and important addresses and a list with passwords (coded) plus a scan of your important documents (passport, personal ID, driver’s license and international vaccination card)
Check if you need to deregister from your home country. For example I have to when travelling longer than three months.
Learn a new language: duolingo.com (free. exist as app as well). It is fun and addictive. You learn the orthography, auding and translating (no speech practices) and the possibility to repeat the learned words.