1. Plan for the travel time. Unhealthy choices commonly start at the beginning of vacation during your commute. Correct me if I'm wrong but most airport restaurants and fast food joints don't have the food you want to splurge on so try to stick with healthier options during the commute.
2. Have yummy food! Vacation is for fun and relaxation. Don't let your "diet" ruin your dining experience at a nice restaurant.
3. Be active. Vacations offer plenty of time for exercise or outdoor activities to burn some calories. Anything you can do to mitigate your caloric excess and promote health while you're on vacation is important!
4. Sleep. Don't forget those 7-9 hours a night.
5. Don't stress. Taking time to relax is really important for your mind and body.
6. Eat a salad or smoothie every day. It's not always possible, but eating in at your rental during vacation is a great way to make sure you're getting a really nutrient (instead of calorie) dense meal every day.
7. Have goals. Going into vacation with a goal in mind whether that be to maintain weight, exercise a certain number of days, eat one healthy meal a day, anything to keep you healthy living focused will be beneficial.
8. Really take time to relax. Rest your mind from all the worrying and "to-do" lists and just rest.
9. Before you leave for vacation, plan your meals and make your grocery list for when you return from vacation. This will save you from continuing your vacation eating for another week after you return.
10. Hydrate with water. Keeping your body hydrated will help your body deal with any insults to the immune system, GI tract, etc.