I went on an all inclusive and only put on 1.75lb, completely off the diet. The only thing I did was to not eat bread and pasta. I ate a couple of pancakes in the morning, and had small amounts of rice. I filled up on fish, seafood, ,salads, fruit, chicken and meat and even ate deserts, and drank what I wanted. I think the answer is to keep really active. This doesn't mean having to go to the gym, not something I'd do on hols. I do lots of beach walking, which is a great work out, swimming in the sea against currents (if you're a good swimmer) watersports and cycling. Just get that metabolism going. I usually lose weight on holiday, because I am so much more active.
Ps I drank beer and lots of Pina Coladas (my favourite)