Hi Heather!
I personally think that even if you are at the Magic Kingdom park from opening to closing you are not going to see everything (and you may find it exhausting). Have a look at the
attractions and
entertainment for the Magic Kingdom, choose several that you wish to experience and if you have any time after that try and see some of your 'second choice' attractions. I would recommend visiting from opening time through the afternoon, or arrive later and stay until evening, or up until closing time. If you are staying at a Walt Disney World Resort hotel you could take advantage of Extra Magic Hours - you can enter the park an hour before opening, or for a few hours after park closing, on select days. Check the park
calendar for opening hours.
Characters at the Magic Kingdom include Mickey & Minnie, and the Disney Princesses at Town Square Theater, and around the park you can find Tinker Bell and some the Disney fairies, and many more. At Disney's Hollywood Studios you can find Jake from Jake and the Never Land Pirates, Cars stars, Lightning McQueen and Mater, Mr. and Mrs Incredible, Buzz, Woody, Jesse from the Disney·Pixar Toy Story movies, Mike and Sully from Monsters, Inc,
and more. As you enter each park, pick up a copy of the Times Guide which will give you details of entertainment schedules, and character meet & greets for the day.