I'm not sure what you mean by "safe", as in causing stomach pain, heartburn, gas or diarrhea?
What I'm actually eating right now, just because I like it but it's an absolute cure-all for me, is yogurt. Store bought stuff isn't as sure for me, as there are too many variables, but I now just make my own using half and half and probiotic powder. Every person who has tried has said it's the best yogurt they've ever had.
I don't eat it plan, and in what I'm eating right now I have some blackberries that were over ripe and smushy, so those were perfect to sweeten it. I also had a failed batch of granola that had a few oats and lots of seeds and nuts but not enough binder, so it was too crumbly (I was looking for toasted coconut chips to put on it, and realized the granola-fail might be even better). That is perfect poured into the yogurt, but I may leave that out if my stomach were grouchy.
I'm also a soup fan, and like different things as a base, then I'll add other stuff. Miso, hot/sour, egg drop, or even Unjury protein that tastes like chicken broth are all favorites to add bits of egg, meat, or a few glass noodles to. I have a thing about baby bok choy lately, but that would be something I'd skip if trying to settle a digestive system.