The most "adult" restaurants are those that do not accept the dining plan. That would be V&A, Bistro, perhaps bluezoo at the Dolphin. I love V&A but if that's out of your budget range, I think Bistro is an excellent place. They have some window tables for 2 that overlook the world showcase, it's a bit quieter than the standard signatures, the waiters are in tuxes, no children's menu offered, service has always been topnotch. This is our "never miss" place for every trip.
If you are looking for a signature that is on the dining plan, I think Yachtsman Steakhouse is one of the quieter ones that has a bit more of an intimate feel to it.