Big picture: Plan to move, and pay attention to your body!
1. Plan to buy or bring some go-to healthy snacks to keep on hand.
2. Eat when you're hungry, stop when you're full, even if it means leaving food on the plate. You're paying to get a meal that fills YOU up, not the most calories for every dollar.
3. Drink a full glass of water before every meal.
4. Be intentional about your food - keep two meals simple, and splurge (calorie and cash) on only one per day. If you can, cook where you stay - one reason I love things like Air B&B and kitchenette hotel rooms - the extra cost is easily made up by using the space to prepare at least two meals per day.
When DH and I were on our honeymoon (which was planned to involve excellent food), we brought everything to make oatmeal and french-press coffee every morning for breakfast (old-fashioned oats, plain greek yogurt, brown sugar, nuts, raisins, cinnamon, the french-press, coffee, cream, and sugar). We were staying in a little condo on the north shore of Lake Superior in June. We planned to eat a really nice brunch the morning that we left. Our lunches were just a simple soup and sandwich from a local shop, and then we'd find someplace fun and/or fancy for dinner. Mixing that in with lots of hiking, and we both ended up losing a little weight!
Honestly, I'd stay away from chains - use this vacation to choose foods you can't get at home! Trying local cuisine can be a great adventure, and a way to discover new things to cook when you get back!
Note to the person going on the cruise:
You're paying to have a great time, not to eat as much as you can! Enjoy all the other things on the ship, not just the food! Choose the foods you absolutely love, choose the drinks you really enjoy (and intersperse with plain soda water with lime!), and leave the rest. Pay attention to how you feel after eating certain foods - part of "enjoying" a food is also enjoying still feeling good afterwards! I can't "enjoy" a doughnut on an empty stomach, or "enjoy" french toast dripping in syrup for breakfast because I just don't feel good from the sugar rush and crash. So I don't choose those foods in those situations, even on vacation! I'll have a doughnut paired with some scrambled eggs after a morning hike, or french toast for lunch after a long bike ride, but not on their own for breakfast!
Take life one day at a time - enjoy today before you worry about tomorrow.