I must be one of the few people who came back from their summer holiday lighter than when I went away. Not a great deal lighter, but lighter none the less.
This must be in large part because we were camping, so I had relatively limited access to food - I couldn't just go and raid the larder any time I felt a bit peckish. Another factor could be sleep - the thing about camping (for me, at least) is that you tend to get up when it gets light and go to bed when it gets dark, which means no staying up into the middle of the night watching crap on telly, which means more and better quality sleep.
There's probably a lesson in there.
Also, I was very moderate in my alcohol consumption - only three bottles of not especially strong beer over the whole two weeks I was off work.