Find out if you can take some of the packets with you, If you can fab. Stick to those and a small light evening meal? Have a good look at the menu and a really good think about the food. You can always ask for no sause on something, or for it to only come with the green veg and no potatoes. If you can't take the packets well its gonna be hard but that will be harder.
I guess you can try your best then carry on as normal when you get home. It will be a challenge, but think about how rewarding it would be. If you can do that you really can do anything. Don't worry too much if its impossible because somethings will be. Just do your best then come back and carry on with it. One slip dose not mean you need to jump off the wagon. Its a hoiladay so alow your self to relax a little. Get food over and done with so there can be more sight seeing/swimming/sunbathing etc.
Depending on where you go, you might find they sell slim fast or something over there. Failing that eat mostly veg I would say. I think for an adult the goverment suggest our diet should be 1/3 veg. So between veg and chicken you should get a lot of the things your body needs, you could also take a muilty vit if you had to.
Good luck

ps I have never had to do that yet, so this is just my thoughts. If you find a good way to do it let me know lol