Alternating days in a theme park and days doing something at a more leisurely pace worked great for us too. A day in a theme park can be exhausting, specially in the summer !
Don't forget Florida offers a lot more than the theme parks. We recommend a picnic at Blue Springs state park, at least one trip to the beach on each coast, Kennedy Space center, St Augustine (longer drive but well worth it), an airboat trip, a little stroll in Celebration and Winter Park (nice boat tour there too), Old town Kissimmee car cruise (saturday night?), some shopping of course... and I'm sure you'll find here many other things to do "on the side"that are fun for the whole family.
You'll have a great time [clap]!