here's something you might try: book a "vacation package" directly with one of the airlines (e.g. usair for a philadelphia to orlando trip, or continental for a newark to orlando trip), but only ask for a "fly drive" package (i.e. without hotel or any of the other extras). i know that usair's website let's you pick this option; continental's may not, but it's available none-the-less. you just have to call.
this is cheaper than buying a normal ticket a surprising percentage of the time (especially if you're not staying over a saturday night). all major airlines have these vacation package arms, and they can usually be accessed directly online from the airline's website. alternatively they all have separate toll-free numbers you can call.
not only are they often cheaper, but for a modest upfront fee (usually about $40 per passenger) you can reserve the right to cancel for any reason and get all your money back (less the $40, of course), an option you don't have with normal discounted fares. moreover you still get frequent flyer points. good luck!