To answer OP:
General admission includes the main Visitor Complex and the basic bus tour to Apollo/Saturn V Center. While the enhanced tours are cool, you don't need them to get a "full" experience. I have only been on one in the seven years I have been an annual passholder.
Thank you. Yeah he’s excited about the Space Shiuttle. We’ve seen two already, so once we go to Kennedy we only have to go out to California. We will go ahead and do the two days.
The Atlantis exhibit is like nothing else. I won't spoil any of it, but we have brought many friends to KSC and adults, men and women alike, have cried.
Two days is worth it. (I should probably mention I live here, my dream job is astronaut, and my fifth degree is in aerospace engineering, so I'm a little biased.) With only one day, you will inevitably miss out on some really cool stuff, but if you only have a single day, my recommendations are:
Do not miss Apollo/Saturn V Center. This is not at the KSCVC. You have to get on a bus to go out to it. The bus tour will take you through NASA property, it takes a good 40-45 minutes to get there. The route the buses take changes depending on what's going on - KSC is still functioning, and there is a lot of work going on. They do generally try to get you around the Vehicle Assembly Building, and at least one launch complex (39A or B.) Try to get on the bus tour early, as they stop at either 2:00 or 2:30. Apollo/Saturn V has an awesome presentation, expect to spend 2-3 hours there. (The ride back is faster.)
Do not miss Atlantis. The presentation is unbelievable, and the bottom floor of the exhibit has a lot of cool stuff to look at. Do the Apollo bus tour first, then the bus will conveniently drop you off right outside Atlantis when you return. Atlantis is 2-3 hours between presentation and all the stuff to see.
If you have to choose between the "Shuttle Launch Experience" and Astronaut Encounter, I highly recommend Astronaut Encounter. Real astronaut, talking about their real missions and experiences. You have the opportunity to get a picture with them at the end of their talk. The launch experience is a glorified carnival ride that isn't worth standing in line for 30 minutes for if you only have one day to be here. Honest opinion, YMMV. You can look up the astronaut who will be there the day you're going, so if you have a mini space geek, they might enjoy looking up some history on the astronaut of the day before they go. Just my personal advice there.
For anyone who is going to be there two days, I recommend this chronological approach:
Day 1
Heroes & Legends (the one thing I am disappointed it changed - this used to be Early Spaceflight, and it was a wonderful exhibit about the US/Soviet Space Race. Some remnants of it remain in H&L)
Rocket Garden (yes, it is worth reading the placards)
Apollo/Saturn V
Atlantis
Day 2
Astronaut Encounter (yes, I promise this really is cool, especially if you have little astronaut hopefuls)
NASA Now
Journey to Mars: Explorers Wanted (kid-oriented)
IMAX (there are usually a few choices, all are good)
Souvenirs and any other odds/ends (they are some smaller attractions that come and go)
Just my two cents on KSCVC.