TL;DR: Not much, to be honest. Life's too short for lots of special stuff.
I'll stick to bottled water and cooked foods over tap water and raw veggies or fish. Much to my wife's amazement, I very rarely get sick, and have only needed to get to the hospital like twice in my adult life (once after a bike accident that broke two bones and once after a motorcycle accident).
I've taken multivitamins in the past, and didn't notice a measurable change from them. Ibuprofen or aspirin are always in the backpack, which are helpful for the occasional hangover.
This is obviously going to vary from one person to the next. Sleeping enough, plenty of water, and avoiding known issues with things like undercooked food cut out a majority of potential issues. Other things like personal hygiene, doing laundry and a daily shower might sound like basic things, but you'd be amazed how many things can fall by the wayside when you're traveling...