For CS locations, you can sub a bottle of water for the dessert to take with you to the parks, or sub dessert for most items listed on the menu board with the "snack credit" purple & white
DDP symbol. Often I can sub my adult dessert for the apple slices or grape packs that come with the kids meals at CS. Those fruit packs are small, intended for children, but are better than eating more sweets I (and my thighs) don't need. Can often sub the dessert for a side that comes with another meal on their menu too. I don't pass on ALL desserts, some of them I like and look forward to.
At TS menu locations often there is some type of fruit cup offered, or a small salad or soup or side of the RESTAURANT's choosing as an alternate to a normal dessert.
At TS buffets just don't pick a sweet dessert, I often go for extra salad if nothing on the dessert table interests me.
As far as beverages, lemonade, iced tea, or coffee are often available as a beverage, as well as milk or juice for children.
Cannot get bottled water to go or use any snack credits at a TS place, they are not set up for it.
Only you can decide if the DDP is a good fit for your family. You are paying for the drink and dessert in the price of the plan, so if not using them you are paying more for your meal than you would OOP. Do the math using average meal prices for WDW, only way to see if a DDP would be cost effective for you.