I have only eaten at two of the places you have listed, so I'll speak to those. You've gotten a lot of good general advice from others. DH and I were both watching our weight last year during our trip. I lost 5 lbs. while we were there! We were not on the
DDP as we didn't want all the desserts and our kids were pretty young so we paid OOP for everything.
We went last May and had dinner at both LeCellier and Cape May. LeCellier was great about subbing sides. I had the mushroom filet (w/out the sauce) and subbed roasted potatoes for the risotto. I've never had it the "traditional" way, so have no idea what I missed taste-wise, but I'm SURE I saved myself TONS of calories. I also ordered a side of grilled asparagus. (split w/DH) If on the DDP, you'll have to pay OOP for it, but it was delicious and healthy. We shared a dessert of their homemade sorbet. It was really good and light. DH had the chicken dish. I don't recall the side, but he enjoyed it.
At Cape May you'll have more options, but also more temptations! They have lots of salads on the buffet, including a "traditional" salad bar type area. I also had boiled new potatoes (plain) and grilled terriaki (sp?) salmon that was very good. Of course, the clams are light if you don't drown them in butter!
I didn't personally try those, but know they're low in calories. They also had some really good veggies on the buffet. The desserts here are good (try the oreo bon-bons and mini key-lime pie), but not so amazing you'll have to have a million. The good part is they are all very small--one or two bites--so you can taste but not blow your whole day. Beaches and Cream is here at the BC, though, so you may want to avoid that area if you have kids!
We found good options at each place we visited. You just have to walk around the whole buffet first (or read the whole menu) to see what the options are. Even places like Beaches and Cream and the Plaza had low fat, sugar free ice cream or sorbet or sherbert on the menu, subbed fruit for fries, had side salads or soup as an appetizer, etc.
You also didn't say where you're staying, but we stayed at the BC and there is a fitness room there. I would usually go early while DH stayed w/the kids and then we'd switch. We did a TON of walking, but you can't beat some good cardio! Even 20 or 30 min. is better than nothing.
As far as CS goes, we had good luck at Earl of Sandwich in DTD. VERY good sandwiches, great tomato soup, and I've heard good salads. (We just came out of a downpour and were DRENCHED so the AC was really cold.) I love entree salads, but needed something warm that night. If OOP, it is also very reasonable. Our family of 5 (kids were 7, 9, 9) ate for about $35--great for Disney. We're looking forward to going back this year!
Sorry so long. Hope you have a great trip and good luck on the continued healthy eating/weight loss!