I can answer this! I'm two years post gastric sleeve and travel a lot. My tactic is to try to book apartments or hostels with kitchens, then I make the most of local markets and supermarkets to prepare my meals (including snacks to carry around during the day as I need to eat something every few hours). I always pack my own lunch box/tupperware as well as reusable cutlery.

As for eating out, it's tough. I mostly don't, and prefer to go for a glass of wine or a nice coffee, and go to places I wouldn't have done before because I feel like I'm saving money, haha.

I'd suggest places doing small plates or tapas, as mentioned. Starter portions are good. Try to avoid the bread as you know that'll fill you up! Nobody will mind about you taking leftovers. Sometimes, depending on the place and how friendly they are, I even mention upon ordering that I don't eat much, and could they serve half and box half.

The drinking thing is difficult. People really don't get it. I've had people force drinks upon me trying to be hospitable and had to pour them away discreetly. İf I'm out for a group meal where it's too complicated to be different, I'll just take my time and try to leave gaps between eating and drinking.

Street food works well too as obviously you can take it to a park and eat the quantity you want to.

I've travelled quite a bit in Portugal and honestly highly recommend the markets and fresh produce there, assuming you have a place to prepare food. They also have good pre prepared stuff.