If you don't want/can't get an ADR for plaza, there is also a hand-dipped ice cream shop on Main Street. We've gotten cones there, but there are sundaes on the menu as well.
Another vote for Ghiradelli's. They're the best on property, IMO. If you order it to go, it gets made right away- much faster than ordering in. You can still have a seat at the tables, or even outside if the weather is nice. There's also a 10% DVC and AP discount.
It was simple, it was fun, it was good...the kid's make your own sundae at Coral Reef. My nephew and I both ordered those and really enjoyed it. All of it too.
If you don't want/can't get an ADR for plaza, there is also a hand-dipped ice cream shop on Main Street. We've gotten cones there, but there are sundaes on the menu as well.