You absolutely can go on vacation and not gain weight. It just takes careful planning. Here's a few tips from our trip to Arizona.
Be as active as you can! Get out and walk. Take advantage of the hotel gym. I walked 80,000 steps in three days. With that kind of activity you can just about eat whatever you want. (but not really)
Plan your meals. Stalk restaurant menus before you decide where to dine. Find places that have healthy offerings.
Try not to eat every single meal out. Bring easy things like oatmeal packets, tuna packets, protein bars and protein shake powder. We would eat these things in the room with our coffee in the morning and then only eat two meals out a day not just saving calories, but cash!
Drink a ton of water. It will help keep you full.
Choose your indulgences. After all, it is vacation. But maybe choose 2 nights to have cocktails and one night for dessert but not both every night. IT CAN BE DONE!