It depends on what you want to experience.
So far no one on this thread has actually suggested a beach that's on the Atlantic coast. They're all on Florida's west coast, the Gulf of Mexico.
If you're looking for wave action pick the east coast - places like Daytona, Cocoa Beach, Melbourne that are all fairly easy drives from Orlando. Cocoa Beach is the closest at about an hour's drive.
If you're looking for more placid waters, fine sandy beaches and the such then the Gulf coast would make more sense. Expect that the beaches that are beyond Tampa will take you about two hours or more to get to. We like Fort De Soto beach in the Pinellas County park of the same name. It's on the Gulf side near St. Petersburg.
You might want to consider spending a day on each coast of Florida.
Dick Taylor