You will want to see the new exhibit for Atlantis.
Take the bus tour which takes you out to see the launch pads and Apollo museum. That will take as many hours as you want it to, depending how interested you are in learning about everything.
Walk out in the Rocket Garden.
You might want to do a lunch with an Astronaut if they are offering it . See an IMAX movie. Go to a lecture. Spend time in the huge gift shop.
There are several places you can eat on property. There are lots of picture ops so make sure you have a memory card for lots of pictures. My battery for my camera didn't make it through the whole day there last time we went and we live about 25 miles from KSC so have been there 100s of times.
On the way to the visitor's center you pass the Astronaut Hall of Fame. You may want to go there too.
Also while in Titusville area you might want to find Space View Park(one of the spots they always reported manned launches from) and look at the memorials there. There are 3 sections-on both sides of the road. It is right on the river There are molds of a lot of the astronauts hands that is always fun to see if your hand matches. It is right on the river so you see dolphins, manatees, Spoonbills, Blue herons, turtles there, too. Oh, yeah, while on KSC roads watch for gators in the itches etc. Might see one in pond right at the Visitor's Center, too.
While in the Titusville area, you might want to check out the National Seashore and do a wildlife drive through the marsh.
If you make it a day and are still there in the evening my favorite restaurant is right across from the VAB building on US1- El Leoncitos. They have an outdoor deck and pretty good margaritas, too.