Make a list so you don’t forget anything.
Usually another country, another currency.
Adapters! How are you supposed to take a picture with a camera without battery.
Check the weather of your destination
confirm all you reservations (like hotel, car ec.)
pay your bills
Don’t forget you first aid kit
Check your passport’s date of expiry
Also take a copy of your passport (If you lose your passport)
Last but not least, have fun and enjoy your holiday