It seems to be really variable on what drinks are included. The last time we went (with free dining,) many people had reported getting milkshakes, etc. on the plan. On my inquiry, none of the restaurants we went to allowed shakes/smoothies as a beverage, and in fact most didn't allow plain coffee/tea as well! Some said that, as the coffee was sometimes listed after the dining choices with the desserts, that it was considered a "specialty" beverage, but some disallowed it without even that much of a rationale. It wasn't a big deal, but it was a little annoying to have to keep using our snack credits to feed DM's coffee habit.
Anyway, to be on the safe side, I'd probably just count on soft drinks as being all that's included and consider anything else a plus, unless you want to put up a fight about it.