I'd just enjoy what you like, be active when you can, fast when it's convenient(e.g., not hungry, wait to eat), and stay as stress-free as possible. You do not have to justify to yourself or anyone else why you want dessert, it's obvious: it's delicious and you're on vacation!
Don't overthink this or let others make you doubt your decisions. Beating yourself up is so unhealthy. Have confidence that you will get back to normal when you return home. No need to believe in advance that it's a downward spiral. That's uncalled for and encourages an unhealthy relationship with food.