I've been worried about this too, which is a shame as I'm about to go on my first foreign holiday in about 20 years!
I'm packing my books although I'm not going to menu plan or anything. I know which foods are free, superfree or fattening. I'm hoping the nice (hot!) weather will make me want more salads than stodge, and that we'll be quite active with walking a lot.
I do fear putting on a lot though as I've found it hard to stay motivated with being on track. It's been hard to lose the stone and a half I have so far, so it will feel like a massive fail if I put on any more than 2 or 3lbs.
I think it's just common sense though at the end of the day. I'm not going to deprive myself but at the same time, I know I won't drink a bottle of wine every night (I'll drink 2! Ah ha ha ha! - kidding! I'd be so ill!).
Hope we all have good holidays and then when we get back, it'll be the motivation to get slim for christmas!