When I'm on vacation, I don't play it like the big hand in poker.
I'm not "all in."
I allow myself to indulge 50% of the time.
That's a huge jump from my day-to-day lifestyle of 80% (very good) 20% (not so good).
Here's how I did it at Disneyland:
1) It's worth it to upgrade to the concierge level. You can munch on low-cal snacks anytime and store some in your room. Plus free bottled water which can save $$$ quickly in the resort.
2) I have a healthy breakfast each morning. Egg white omelets with lots of veggies. Fruit. No wasted calories on OJ or pastries. I try to be full when I leave and satisfied knowing I've banked a lot of calories to spend later in the park.
3) I always pack almonds. A handful goes a long way when you're starving and about to attack the corn dog cart.
4) I consult the guides and scout the best options for clean food. This butter lettuce salad and ahi tuna with Asian slaw is my new find at Stage 12 in Disney's California Adventures.

5) My absolute favorite food is the vegetable kabobs at the Bengel BBQ. It's a small stand across from the entrance to Indiana Jones.

6) I love hot dogs! I get the kids meal because it's smaller and they are turkey instead of beef. That's a lot of calories saved from the standard adult version.
8) My pet picks for lunch and dinner in the Magic Kingdom: Big Thunder Mountain BBQ, Plaza Inn for the broasted chicken, Cafe Orleans for the salad with grilled salmon, and Rancho del Zocalo for soft chicken tacos. I adore the Blue Bayou next to Pirates of the Caribbean, but I always forget to make reservations and I only wait in line for rides, not num nums.
9) Napa Rose at the Grand Californian is a destination restaurant. Don't dismiss it because it's in a hotel - this is world class dining! Chef Andrew Sutton cut his teeth at Auberge du Soleil.
The wine list has over 17,000 selections and the wait staff is incredibly well versed with it. I prefer to order two light appetizers instead of the heavier full-size meal. Especially if you're riding after dinner :) My world was rocked when I feasted on this ahi tuna carpaccio with blue crab and lentil caviar.

10) I'm not much of a sweets person, but if I am going to treat myself, I head for the fritters at the Mint Julep bar in New Orleans Square. They're very close to an authentic beignet and can't be beat when they are hot and puffy!
11) The lines for food cues are usually shorter on the left. Attractions, too. The only exception is the entrance to Disneyland. The parking lot tram lets everyone off in Downtown Disney. All those peeps are approaching the park from the left and always head to the closest gate. You'll get in much faster if you use the entrance furthest to the right.
I can spend four days at the Happiest Place on Earth and come home without gaining weight.
Lots and lots of walking of course plus sticking to my 50/50 plan.