I know a few places, like Le Cellier, for instance, have a special complete dinner deal... like appetizer, entree, & dessert for a fixed price, and if you choose that type of meal, your options for the included appetizers and entrees and desserts may be limited, with no substitutions... but you're also free to just forget the appetizer (or pay OOP) and just get whatever entree & dessert on the menu, instead of for that particular complete dinner deal.
And if you eat at Capt. Cook's at the Poly, you can have self-serve dole whip!
For the most part, if there's a snack that's under $4 and a single serving, it should count as a snack credit.
You'll see the little purple DDP square on the menu, indicating that it's on the DDP as a snack, and if you're not sure about something, it doesn't hurt to ask!
Here's a fairly-regularly-updated list of snacks obtained on the DDP this year:
2009 Confirmed Snacks included in the Dining Plan - The DIS Discussion Forums - DISboards.com.