I have to be really careful with snacks - I have a tendency to mindlessly munch away without thinking about it. Couple that with a tendency to drink a lot of Irn Bru, and I found I put on a lot of weight a few years ago. So, these days I eat at mealtimes, and try not to eat anything in between times. Also, I only drink water between meals, and indeed at most meals.
Since I travel to the game directly from work, I either order in a pizza or I bring sandwiches. It should be sandwiches all the time... but I don't always live up to my good intentions. As I'm the DM, the game will either wait until I've eaten before it starts, or we will take a break when the pizza arrives. I can't game and eat (at least, not 'proper' food) - I have too much else to do.
We tend not to have alcohol at the game, and I usually have to drive anyway so would have to abstain. However, we often have a Christmas game for which I use public transport. On these occasions, I might have a few.