Gonna 2nd all of the above...and mention that Seaworld has a 3 pack park pass and a 14 day park pass that allow admission to SeaWorld, Aquatica, Busch Gardens, and the Tampa waterpark for $109 and $139/person, so you can have a lot of fun at kid-type parks for not a lot of money.
Sealife is the other referenced aquarium. Crayola and
Legoland are both great ideas and really not expensive. If you have a science museum pass already, you can do the Orlando Science Center (I wouldn't pay the admission fee for it, but free is an awesome price for it

. There are trampoline parks (and bounce houses) all around the area that are good for everyone. There is also Dinosaur World, Gatorland, Green Meadows Petting Farm, and Central Florida Zoo, depending how much your child likes animals and what types

.
And there's the beach if you're willing to drive...
It really depends if you're looking for "down day" activities or "get up and go" long ones...