As a space nut, with a DH also a space nut, I’ll try and answer.
Kennedy is a two day destination. So if you are planning one day you need to be selective. It’s had a massive make over in recent years.
Decide what you want to see. Are you inspired by the pioneer astronauts, the Mercury 7, the moon missions? The Astronaut Hall of Fame is now onsite and in a separate, magnificent building. Some interactive stuff, some memorabilia.
The rocket garden has replica Mercury and Gemini capsules to (try) to climb in, some early rockets etc. The astronaut memorial mirror is sobering; I got very emotional the trip before last because I didn’t go over to spend a few minutes there.
Clearly Atlantis is a must-do. And it’s “on site”, at the space centre itself. There will be a queue to get in.
Then there’s the IMAX; yep, cool to see something space related on a screen like that but, we’ll it’s skippable for a first visit, unless you really have little interest in much else.
There are various hand-on type exhibitions, Mission to Mars for example, if that’s more your thing, the future of space exploration.
And the simulator, which is actually somewhat underwhelming if I’m honest. Especially if you’ve done Mission Space! Yes, the seats go back and the hatch doors open giving you a view of the earth up above you… Don’t get me wrong, it’s fun, but with one day not essential.
And then there’s the Tours. The Explore KSC is the standard one. You go past or near the VAB, it’s huge, last one of the Launch Pads and to the Saturn 5 building. You start with a simulated mission launch from mission control, then go to the actual rocket. Impressive. The tour takes 2 hours, so you need to fit stuff around it.
However the Early Space tour takes you out to the original mission control room which is behind glass, from the days before it moved to Houston. And also past some of the early launch sites. I haven’t done the new updated tour, but DH and I did it many years ago on our first visit.
Dine with an Astronaut, well the food is meh to be honest. We did one with Al Worden, Apollo 15. Buffet meal, sitting at tables of about 10. The Astronaut speaks for a little while about his / her missions and life afterwards, then question time. I had a bundle of them, but settled on one. You do get a photo with them, and a chance to speak to them a little.
My suggestion for one day:
One of the Tours - whichever inspires you
Hall of Fame
A stroll around the rocket garden and 10 minutes to pay respects at the memorial.
I also recommend the website, the KSC one not NASA. They actually have a tool a bit like TP. It gives you the different things to do, with a time estimate and you can build an itinerary. And it also has suggested itineraries for a one or two day visit.
What I would also say is that you can upgrade your ticket as you leave to two days for very little, I think it was $15 two years ago.
Or just plan to go back again next time and do some more. I could not ever visit Florida and not go. And it’s why we keep deciding on WDW and not DL.
Feel free to ask questions. Go look at the website first though!