"The world is a book, and those who do not travel read only one page"

Before you travel ensure that your passport is valid for travelling, most countries require at least 6 months till expiration in order for you to enter. Some countries also require Visa's before you travel to them, make sure you do this before you travel (these also have a cost to them).  

Before you leave download any GPS maps for locations you are going to in case you don't have WIFI or cellular. If you are doing a driving holiday or renting a car see if there is a GPS Audio tour for your destination (try Guidealong).

Getting an expensive phone bill at the end of your holiday is never great, so don't forget to turn-off data roaming and if your on a cruise go on airplane mode. If you are visiting lots of overseas destinations consider an  E-Sim.

Check the local currency of where you are going before you leave, some countries may not take-overseas cash or if they do make sure you have small denominations as they may not give change. You can often order this before you leave or use local ATM's where you are going (for a fee). Alternatively look at getting a credit card that doesn't charge international fees to save money.  

Ensure you have international travel insurance that covers the places you will go, the things you will do (ie. cruising, snow activities etc.). If you have a credit card read your PDS to see if you have Travel insurance included. 

Vaccinations may be required for entry into some countries. Make sure you check with your doctor what vaccinations or other preventative health care measures are recommended for where you are going.  

If you have a long flight and want to try and get lots of sleep on the plane consider investing in a good quality eye mask and ear plugs to block out noise and light. Also consider taking a headphone adapter to use your own headphones.