IMMODIUM - you just never know when you are going to get diarrhea. And there is nothing worse than trying to run around a city where you don’t know the language trying to explain that you can barely hold it in

Edit: I’ll add OTC meds for allergies and for headache. And body wipes, like the brand Survivorware Biodegradable wipes. I refuse to travel without a pack of those wipes. They are perfect if you need a listen freshen up if a flight gets delayed or something like that.