Tifa's Piano
Upon obtaining the Highwind, return to Tifa's and Cloud's hometown, Nibelheim. Go to Tifa's house and go upstairs to her room. Examine her Piano and play this melody: X, Square, Triangle, L1 and Triangle, L1 and Square, X, Square, Triangle, L1 and X, Circle, X, Square, X.
After playing it, you'll recieve Tifa's Level 4 Limit Break, Final Heaven.