The day you go to Magic Kingdom, I wouldn't go on a Sunday. I would go straight to Space Mountain first. The wait at opening will be minimal. It might say 20 or 30 minutes, but in our experience, it's only 10 or less. If you really like it and want to do it again, I would get a fastpass when you get off. If you don't feel you want to do it again, then save the fastpass for another ride. I would then go to Frontierland and ride either Big Thunder Mountain or Splash Mountain. They are side by side. If you don't want to get wet, I recommend you save Splash for later in the evening and use fastpass. If you didn't fastpass Space, and Big Thunder has a long wait--it shouldn't this early-- then fastpast Big Thunder. When you finish either or both of these two, I would go to the Haunted Mansion. The queue is usually pretty fast even when long. Next, I would go to Adventureland and do Pirates of the Caribbean. By this time, if you had to fastpass to anything, it will be time to go do that ride. Your fastpass will have a time when you can get another fastpass. Make sure and watch for that time and get another fastpass for something else you may be interested in doing. We always fastpass Jungle Cruise. Once you've done Space, Splash, Big Thunder, Haunted Mansion, Pirates, and Jungle Cruise the rest is icing on the cake. The "kiddie" rides are really enjoyable also, but for two 23 year-olds, I would save them for last. Mickey's Philarmagic is a must do. Pooh and Peter Pan and Small World are fun. Unless we get there really early or it's not a crowded day, we fastpass Peter Pan and Pooh. We never fastpass Small World. Buzz if fun, but nothing like Toy Story Mania over in Hollywood Studios. If you like nostalgia, then Carosel of Progress is also a must. We did it three times last week. We finally rode the Riverboat. That one is fine, but not one I want to do again. You can do the Peoplemover at any time. There is never a long wait. Remember to use the fastpass as often as you can. It will tell you on the fp when you can get another. You will have a one-hour window in which to do the ride. You can come back at any time during that hour.
I would go to the Disney World official website and look at the park maps. Get familiar with where each land is located. MK is basically a circle. There are several cut throughs that will help save time. When you get to the end of Main Street by the ice cream parlor turn right and take that path to Tomorrowland. Turn right again at the end of the path by The Laugh Floor (another good one for evening). You will be in the area where Space Mountain is located. It's to the right after you pass the Astro Orbiter loading station.
Don't hesitate to ask any cm (worker) how to find something. It will save you tons of time. Once you been around the park once, you will understand the layout.
I have to go right now, but I'll get back on tomorrow and tell you what I'd do for the other three parks.