Probiotics, in the form of yogurts, kefirs and fermented vegetables and teas are excellent for stomach health. A serving or two a day is very healthy.
Probiotics are a great way to keep your gut flora healthy and strengthen immune system strong.
Dr. Natasha Campbell-McBride has written extensively on the subject of GAPS (Gut and Psychology Syndrome) and how important it is to one's overall health to have healthy gut flora.
I would, however, not recommend Activia as a daily source of probiotics. Most brand-name yogurts contain large amounts of unnecessary sweeteners and unnatural ingredients. Opt instead for full-fat organic yogurts, or even try making your own yogurt or kefir.
There are many other ways to take in probiotics; the variety is actually quite diverse. GT's Synergy kombucha tea is extremely healthy and good for the stomach, and kombucha is very easy to make at home.
Even quality sauerkraut or Korean kimchi can have great probiotic benefits.