I suggest you stay at the Caribbean Beach Resort and play dumb later. "Well, the name beach was in the title -- I was sure there'd be a beach."
Seriously, it'll take you over an hour just to get to the beach. If she's dead set on it, I'd suggest that you make it the last day of the vacation, and do it as you leave Orlando. Spend six days at Disney, then spend the last night in an oceanfront place. I don't think it's the best plan in the world, but it'll be less intrusive on the rest of your vacation.