you don't have to say no [or yes for that matter] to everything. look at what is available and pick what you think you would like the most.and have some of it. also make an effort to note portion sizes when you are measuring. in other words, when you are home, don't just weigh out your ounce of cheese, look at the size of that ounce of cheese. compare it to a finger [yes, i know that a pair of dice is about the size of an ounce of cheese, but do you carry dice around with you? or baseballs? or any of those other handy a portion is this size items? i sure don't.] and try and keep that in mind. i know for me a balled fist is about a half cup and what i can hold in my cupped hand is about a third of a cup. your hands might hold 2/3 cup or half a cup. depends on the size of your hands. but if you pay attention to what you serve yourself you can get better at eyeballing in situations where you don't want to bring scales, cups and spoons. if you think you might ever want to not track, having a good grasp of what a portion size is is a helpful trick.
and just because you are on vacation that doesn't mean it's the last time you'll ever be on vacation or have the opportunity to eat those items at those places again. in other words, you can't think of every meal as a once in a lifetime, last chance opportunity. if your great aunt kathy is in failing health and she made her signature dish, by all means have some if you like what she makes. that's a reasonable last chance at something. but if you're going to chain restaurants, having storebought food from chain supermarkets, the same green bean casserole that susie makes every holiday, those sorts of things it's fine to skip or really skimp on. even if it's a local fancy restaurant ask yourself if you really couldn't get something similar at a fancy restaurant at home? it's fine to indulge in those special splurges, you just have to be very discerning about what a special splurge is. in other words, every food you will encounter on your trip can't be a special splurge.
and the easiest way to deal with pushy family is "thanks, i had a little of x, it was excellent, but i am stuffed right now." if it's a situation where everyone brings good food, make a fuss over those foods, make sure you try a little of everything and just be insistent that you are full and couldn't possibly eat another bite.
-google first. ask questions later.