Also there are a couple of places to actually swim and a load of 'visit dolphins in the wild' trips from the Keys, but you'd have to take off for a couple of days to do those. In fact you'd probably be pushing it to do the Miami seaquarium in a day too.
Perhaps you'd better use Discovery Cove as an excuse to return to Orlando next year (unless they have last minute cancellations?)
ritzcarlton.com You'll have to book the room as well, but for one night only. Probably easier than driving elsewhere and you would get to check out the resort.
They hold blocks for the swim. I checked sveral days around your schedule and it was available. You can also contact the hotel directly for further info. The concierge there is outstanding. They bend over backwards to accomodate.
The Ritz-Carlton is an exceptional property, well worth trying. Barring that, there are a few other options:
International Divers has a Swim with the Dolphins tour, but it's included in a 2 day tour down to Miami and Key Largo. Check http://www.swimdolphins.com for more information.
Closer to Orlando, but not involving dolphins, is the lovely Crystal River (on the Gulf Coast). Orlando Dive and Snorkel Tours does a Manatee Snorkel Encounter for only $40.00. Try http://www.diveorlando.com for more information.
