Cinderella, Belle (in her gold ballgown), and Ariel (in her wedding gown) ride with their repsective Princes on the "A Dream Is A Wish Your Heart Makes" float in MK's Share A Dream Come True Parade. Snow White also appears in the parade on her own float. Ariel, Cinderella, and Snow White are also featured in the SpectroMagic Parade at MK and Snow White, Ariel, Jasmine, and Princess Leia (not a Disney Princess but a Princess none the less) appear in the Disney Stars and Motor Cars Parade. Also, if she's a Princess fan, be sure to see the Cinderella's Surprise Celebration Show at MK (Cinderella, Snow WHite, and Belle appear with their Princes in it), Fantasmic at MGM (Snow White, Ariel, Belle, and Pocahontas appear in it with their heros) and the Beauty and the Beast and Little Mermaid shows at MGM.
As for meeting and greeting the Princesses, Town Square at MK (Princess Aurora is usually there and sometimes a few others), Toontown Hall of Fame (Cinderella is always there), Ariel's Grotto in Fantasyland (Ariel, of course), near Small World in Fantasyland (Snow White), In Adventureland near the Magic Carpets (Jasmine), the Expo Hall on Main Street (I usually see a few Princesses here sometime between 11 and noon after they finish the breakfast at the Castle), Germany, France, Morocco and the International Gateway at EPCOT (Snow White, Belle, Jasmine, and Aurora are usually there respectivly), and the Sorcerer's Hat at MGM (Snow White, Belle, Jasmine, and sometimes appear there in the afternoon) are good places to look for them.