You have gotten WONDERFUL replies back. Not sure that I can do any better, but here goes. Remember... this is just MHO, of course.
You MUST have a plan BEFORE leaving. Like Lisa said... will it be a "totally off plan" vacation, or a "one splurge per day" vacation, or WHAT?? YOU decide what will work best for YOU! Do you want to work when you get home at re-losing pounds, or are you hoping to return home at the same weight as when you left?
Before our last trip to WDW in 2008, I was BACK on WW and had lost about 35 pounds (but still had plenty left to lose). I was bound and determined to enjoy my trip, but not go CRAZY. I kind of combined the "eat half" and the "one splurge per day" ideas.
Most of the time I stayed as OP as I could... I shared meals with whoever in my family would with me... I ate the kids healthy side dishes that they didn't always want... I ate LOTS of veggies and fruit and salads and water. As much as I LOVE Disney, I don't find anything special in their most of their counter service foods... their burgers, dogs, and fries aren't special enough for me to consider splurging on them... even if they are DISNEY!

So they were easy to avoid. (You can find healthy foods at Disney if you really look, I promise!) If I really wanted a Mickey bar, I would have one, but most of the time just a bite from DD's or DS's bar (or one bite from each!!) was enough to satisfy me! When I was craving my "one per trip" caramel apple, I had it, without guilt!
When we ate out (just a few table service meals), I only ate what was WONDERFUL. When at Chef Mickey's I started with a nice full plate of green salad... light on the dressing. Then I moved on to the "real food"... I took a small portion of everything that looked delicious. If it was TRULY delicious, like the mashed potatoes and prime rib, I ate the entire portion. If it wasn't, I left it behind. I did go back for seconds on a few items that were just superb.
Same for desserts. I took a trip through the buffet and picked up a few things that looked just wonderful. I took a small nibble of each and if it was truly great, I ate it (like the banana bread pudding!!!). If not, I left it on my plate (the oreo mickey ear things... ICKY!). And I only tried things that are really special... soft serve ice cream and brownies I can get ANYWHERE. While at Disney I try to eat the "special" stuff.
I left Chef Mickey's (and most all of our meals) feeling pleasantly full, but not stuff and NOT GUILTY!!! No regrets at all about those meals. No regrets about what I DID eat, and certainly no regrets about what I passed up. Here it is, two years later, and I can remember the stuff I ate and enjoyed, but I don't even remember what I passed up.
I didn't do any "real" exercise, but anytime someone needed to run across the park to fetch Fastpasses or get something from the locker or to grab someone a drink or whatever, I did the running. I can't even IMAGINE how many miles we logged every day. This year when I go, I WILL be doing regular exercise, at least two of the four days we are there.
And the morning after we returned it was STRICTLY BACK ON PLAN. I can't remember if I put it in writing, but
that was the MAGIC for me! Sure, I could have eaten whatever I wanted while away and dealt with the pounds after the fact or eaten OP and not had those pounds to deal with when I was back home. But EITHER WAY... it wouldn't have mattered in the LONG RUN if I didn't get right back on plan upon returning home!!
An extra day off turns into an full week off.... a full week off means I don't want to hit the scale. Miss weigh-in a few times and suddenly I am on a slippery slope back to 220 pounds I used to be. I can ASSURE you that I NEVER intend to see that number again on my scale.
Put your plan in writing... put it on the fridge, on the cupboard door, the front door, the bathroom mirror, the treadmill, the steering wheel of your car, your computer screen... any or all of these places.... a date/time/meal of when you will be BACK ON PLAN!! If you're returning home late in the day, the most obvious choice would be to start right back the next morning. If you are returning on a early flight and will be home by lunch time, maybe dinner should be your first "back on plan" meal. If you are a regular exerciser, plan your exercise for the entire week of your return... date, time, type.
Plan ahead and have some healthy meals cooked up and in the freezer, ready to go when you get back.... so if you don't have time to hit the grocery store right away to restock the fridge, you still have healthy stuff available and have NO EXCUSE to have to continue to eat out or order in take-out food. Leave your food journal out on the counter so you see it when you return home. (Take a travel food journal with you if you think it will help!!) Tell others about your plan to get back in your healthy patterns when you return and hopefully they can support you.
PUT IT IN WRITING AND COMMIT TO IT... whatever you plan is. Share it here with us if it helps.
Thanks for reminding me that I need to do this for my upcoming Mother/Son trip! I hope something I shared here helps...............P