After completing the game with all six characters, you can race in Twinkle Circuit with anyone. Simply choose the character you want and go to station square. Once there, go over to the little patch of grass and weeds by speed highway entrance and pick up the ticket. (It will be rotating and thus easy to spot.) Then, go into the Twinkle Park building and, instead of entering Twinkle Park, enter Twinkle Circuit. You will hear a special noise that tells you that the secret is unlocked. Repeat the process for all the characters.