That is a great question!
In my experience, the last thing people want to do is lug a big bag of supplements on holiday or vacation. (See my previous post, Am I Taking Too Many Supplements?) And there is some evidence that we respond better to supplements if we pulse them, in other words, take supplements, then stop for a while, then resume taking them. If this is true then why not take a personal holiday combined with a supplement holiday?
On a more serious note, there are some important supplements that I would recommend if you are traveling to a less developed area of the world. In these places you will be undoubtedly exposed to various "bugs" that, while harmless to indigenous people, may make you sick. I find the best preventative measures include avoiding organ meats, raw shellfish, undercooked foods and of course local water sources. But, even if you are just going to grandma's for a few days, a good preventive measure is to supplement your gut and help you balance the good flora with the stress of traveling and unfamiliar food.
Supplements to Take When Traveling:
First, consider taking a good digestive enzyme that contains plenty of betaine HCL. Take this whenever you consume any food and the extra acid will help not only digest the food but also help digest the "bugs" so that they can do no harm.
The second supplement for digestion would be a good probiotic. Taking probiotics helps ensures that you "crowd out" any bad bacteria that might survive the stomach and be passed into the intestines.
The third supplement is only needed if the other two aren't enough and you end up with "Montezuma's Revenge". Consider carrying a bottle of activated charcoal. Charcoal will effectively absorb the offending bugs in your digestive tract.
Melatonin can be helpful if you have trouble falling asleep, or some people use it to avoid jet lag after a long flight.
Should you get sick, Zinc or Vitamin C supplements can help lessen the duration of a cold if they are taken right at the start of symptoms.
There is wisdom in the saying, "an ounce of prevention is worth a pound of cure".
Being sensible about what you eat and drink is your best defense. Enjoy your vacation and send all of your usual supplements a post card!