How can I shift holiday weight gain?

I have just come back from holiday and find that I have put on half a stone! The thing is, I don't know how.

I always eat less on holiday because of the heat and exercise much more with swimming and walking around sightseeing. I do drink a bit more, but never to get drunk.

Have you got any quick tips for shifting the extra weight so I can show off my tan while the weather is still nice over here? Thank you!