Tons of stuff to dive in the N Fla area even as Open Water divers. Worth repeated trips down. I know, I've done it. Flown down, drove down both.
If you don't have a copy of Ned Deloach's Underwater Florida, I recommend getting a copy. Think it's in its 11th or 12th edition.
Amazon.com: Diving Guide to Underwater Florida, 11th Edition (9781878348395): Ned DeLoach: Books
Ned lists tons of dive sites throughout Florida, broken down by area.
Other thing I would recommend is getting hooked up with American Pro Diving in Crystal River. Guided dives available and some good people there as well.
Ginnie Springs offers a couple great dives between the ballroom and the other side with the Ear and the Eye. Lots to poke around and see. It can be 3 dives depending upon what you do and your air consumption. In all the times I've been there I've not done the drift between the run from the Ballroom and the Ear / Eye.
Blue Grotto is fun for a couple dives easily and Devil's Den is right up the road from that. Lots of fun.
Manatee Springs is good for a couple dives as well but as was posted, no tank rentals and fills available. It's a park.
What else to do?
There's Weekeewatchee Springs, King's Spring manatee snorkel dive, Rainbow River, Paradise Springs (with a guide)... might want to consider taking an AOW+Nitrox or Cavern Class while you're down there... great way to get guided dives in these places plus training all in one package.
All in all, there's stuff to do there for many days of diving in the area.
Oh and if you're flying into / out of Orlando, don't forget that Epcot offers a dive in the Living Seas exhibit!