Soda Inflation

Well, to start off, most of the research and testimonials I have found say that the coke and mentos things internally dont usually work because the mentos "holes" (that react with the carbon dioxide) usually get destroyed or coated by any amount of saliva, so swallowing it wont work. You might get a little, but generally not. 
And the issue is that even if it does work, the stomach isnt supposed to be stretched normally more than three litres at a time, so there could theoreticallly be too much pressure that before you could vomit could cause a tear in the stomach.

Now, if you want distention, as mentioned above, I find that anal inflation with either air or water works best; they typically yeild similar results. The difference is the feeling from each, with water being a heavier stretch and air being lighter. Moderation is of course key, as the intestines can only safely hold about 1 PSI of water or air, but the general adult can safely hold about 4 litres of water or air. However, do not expect to contain that much the first few times as the large intestine and rectum isn't used to that, so it can take a few times to get some noticable distention. Never try to push yourself, that doesn't work, and you can end up rupturing your body. Remember, no fetish is worth dying over just for a risk.

I find using an aquarium pump is the nicest and most relaxing way to inflate, although you can use a bike pump or something with really low pressure.

For water, go with an enema kit, because you can add salt (one teaspoon to each litre) to the water to make it isotonic. The benefit to that, is that the water won't be absorbed in the intestines, nor will water leech out of the body into it, so you can safely retain the water and not risk water intoxication.