In a surprising turn of events, I'm likely going to both DL and WDW in the same month (October), but less than 3 months out from RNY. I have been researching this exact thing.
1. Bring a shaker bottle, and protein powders. Get cold water/milk around the park for your shakes, and hydrate with protein if you can. December won't be too hot, but still likely to be more active than you are regularly used to, so make sure you are consistent with your hydration.
2. Your Surgeon is likely to give you a medical card (or one can be purchased online) that advises of your food restrictions due to surgery, bring that with you, Disney is usually really good about accommodations anyways, but it doesn't hurt to have back up.
3. Speak to the chef if needed. The Chef's at Disney would be more than happy to work with you to find a solution that meets your dietary needs.
4. HYDRATE HYDRATE HYDRATE!!!!!
5. Start your day with your protein shake in your hotel before going to the park.
6. AYCE/ Buffets won't be worth the price in food. Disney generally will still charge as adults, but it does allow you freedom over what you put on your plate. Is the ambience worth it to you?
7. Outside of the AYCE places, dont be afraid to bring a tupperware to toss some leftovers in, and bring with you throughout the day (obviously depending on shelf stability/if you have an ice pack). This will allow you to get the most out of your meals, and still have more protein if/when you get hungry later on.
I will update with more after my trip, and wish me luck!!!