In general, you can order any non-adult beverage, entree and dessert (and appetizer if on the Deluxe plan). Some locations have appetizers or entrees that specify "for two" on the menu; these items must be shared as the selection for two Disney adults on the plan.
Also, I believe a few non-Disney-owned restaurants may have an exclusion or two. For example, I know Via Napoli excludes specialty non-alcoholic beverages such as the acqua fresca, but I'm not sure which if any signatures have similar policies.