..Hm, if I remember correctly, you can't. Depending on the whose story you are doing, some areas are blocked off and some areas are open to you. You won't be able to reach the top of Rainbow Falls until Aqua where, due to storyline purposes, "Booster Orbs" are added to the side of the cliff.
The only purpose that Rainbow Falls has for Ventus is a sticker and some chests. (: