If you're a typical adult visiting the Magic Kingdom, you're likely to be bored. Most of the Magic Kingdom is designed for the pleasure of kids and unusually cheerful adults who sing along to Disney tunes in the car on the way to work. The reasonably exciting rides like Space Mountain and Splash Mountain can have waits of up to two hours. In this writeup I'll talk about ways you can make your experience at the Magic Kingdom more adventurous. My knowledge comes from working at Splash Mountain for two summers before being fired for safety reasons.
The key thing to keep in mind is that there are thousands of employees at the Magic Kingdom and tens to hundreds who take shifts at every attraction. Everybody doesn't know everybody--in fact, nobody knows anybody. In addition, Disney employees regularly stroll around the park any time for free. They get free, convenient parking in the employee parking lot. In plain clothes, they can go into employee-only areas like the "tunnels", which i'll discuss later, without anyone thinking twice.
Note: Disney employees are not allowed to have facial hair, facial piercings, visible tattoos, unnaturally-dyed hair, etc. If you look very weird you cannot pretend to be a Disney employee.
The Magic Kingdom has many "backstage" places where employees take breaks, eat, etc. Most prominently, there is a huge underground tunnel system that connects all of the Kingdom to an employee cafeteria, dressing rooms, payroll offices, the bus to the employee parking lot, breakrooms, and more. This is a really cool place to visit for your first time. There are tons of entrances to the tunnels scattered across the Kingdom. The main purpose for the tunnels is to prevent people dressed in costumes from one land, like Frontierland, from walking through another land, like Fantasyland, to get to work. Supposedly this spoils the show.
In Frontierland there is an entrance to the tunnels through the double-doors by the restrooms in Pecos Bill's Cafe. Walk through two sets of double-doors, hang a left until you see an elevator, a breakroom, and a stairwell. Take the stairs down to the tunnels. Cinderella's Castle has easy access to the tunnels as well--the stairs down are located through a door on the side of the castle. There are many other tunnel entrances but I have difficulty remembering them all.
So what do you do when you get inside the tunnels? Well, the tunnel system is huge and even employees get lost in them. Just walk! No one will bother you if you don't act like a fool. Nobody would think twice about giving you directions--they would just think you're a new employee. Walk around, check out the employees half-dressed in their costumes, and explore! There's a cafeteria where the food is far cheaper and healthier than it is for customers. Really this is quite safe. I advised my mother and her friend to do this and they had no problems.
If you get bored exploring the tunnels and you're really looking for some mischief, the dressing area is the place to go. Here there's a huge bin with dirty costumes for which you might find use. If you want a clean costume you usually have to have an employee ID. However, it is common for employees to find that their checked-out costumes don't fit, and to exchange them. You don't need an ID for exchanging. What you do with your costume is up to you but I'm sure you can think of something.
Would you like to ride Splash Mountain or another ride without waiting in line? Most of the exciting rides have a "baby-swap" policy that you can exploit. The entertaining rides at the Magic Kingdom have minimum height requirements. Kids too small to ride have to stay with some of their family--that's YOU--while the rest of the family waits in an hour-long line. When the rest of the family finally finishes the ride, they watch the baby. Typically, YOU then walk up the exit and say sort of unsurely, "We're doing the babyswap thing that the girl at the front told us about." You don't need a pass or anything. Take advantage of it.
Splash Mountain is a very complex attraction that has tons of neat backstage stuff to see. There are many exits to the backstage area there--some say "cast members only" (Disney likes to pretend its employees are actors and actresses), others are unmarked. There's a machine shop where mechanics work on the boats. Best of all, up some stairs there is a control tower from which employees monitor cameras to make sure nobody is jumping out of his our her seat. It would be fun to go upstairs, have a peek...ask if "Susie" is around...pretend you own the place. Just remember, you're a Disney employee and everybody doesn't know everybody!