First of all, you have to decide if you want the Atlantic coast or the Gulf coast. The Atlantic is slightly nearer, but IMO the Gulf coast beaches are much nicer and the water is actually warm!
If you decide on the Gulf coast, there are several beaches on a fairly short stretch, all with their own merits. If you take Fort de Soto beach as a starting point (it's just South of St Petes beach) and work your way North you have...
Fort de Soto beach - rated the #1 beach in the whole of the USA.
St Petes Beach - only a couple of miles north of Fort de Soto.
Treasure Island - just a little further North again, and supposedly a hidden gem
Indian Rocks beach - another beach that's very highly spoken of
Clearwater beach - famous for its beachfront cafes selling grouper sandwiches, and plenty of watersport opportunities
These are all around a 1.5hr drive from Orlando
