A week of sun and sin is over. You met a nice girl, you partied together, had fun together and enjoyed life together. For a few days. Now it is time to head back home and unfortunelately your home is in a different part of the country, or even the world! Do not be one of these fools that think that "long distance relationships may work for us." Accept a holiday relationship for what it is: Some fun in the sun for a few days.
Do it under 4 eyes
You and her alone, away from your friends or other people that can disturb you. A walk along the beach in the sunset is perfect.
Do it quick
Be a ninja, act fast and decisively. Do not spend a lot of time talking about other things when the reason you are together is that you are going to break up with her.
Keep It Simple, Stupid. Tell her with great regret that you will have to end the relationship. Do not offer to call her, be friends or any of those other cushions that can make it easier to break up now, but a pain in the ass later on. To break up in stages is never a good idea. Not for you or not for her.
Be prepared for tears
It has only been a quick holiday fling, but you never know. She may become devastated and cry her heart out. Have a napkin ready, let her cry on your shoulder for some minutes. But that is when it has to stop.
Do not change your mind
Do not get tempted to change your mind. It is not what you really want and the only reason you are considering it is because you want to be kind to her.