The only place where Ariel does meet-and-greets right now is at Ariel's Grotto in Fantasyland. She appears there usually from 10am to sometime in the evening every day.She does appear as a mermaid so when the weather gets bellow freezing, she is relocated to Toontown Hall of Fame which is indoor and heated in the winter. When you go meet her, I would recogmend doing so as soon as the Grotto opens. Since this is the only place where she does photos and autographs at WDW, her lines can get very long quickly.
As for parades and shows, she's also in Share A Dream Come True Parade at MK (as a human in her wedding gown), SpectroMagic at Mk, Voyage of the Little Mermaid show at MGM, Disney Stars and Motor Cars Parade at MGM and Fantasmic at MGM.