As others have said, the best place to see the princesses is Fantasy Faire (usually three princesses inside the building, but they do vary so you can check the sign board outside to see who is there, and often one outside by the bridge (usually Rapunzel or Merida)) and the best place to see the fairies is Pixie Hollow (Tinkerbell plus a friend usually). If you're willing to spend money, there is also Ariel's Grotto (breakfast and lunch) which features princesses.
For the boys (and girls), there is also the Marvel meet and greet in Tomorrowland (usually Captain America and Thor), Aladdin and Friends can often be found in Adventureland, and (right now) Jack Skellington and/or Sally can be found in New Orleans Square.