Chiron and Mr. D had given you and Grover permission to go out and have fun after a successful quest. You both had pleased the gods after being able to return a small belonging of Apollo's. So, since you and Grover were allowed to go anywhere at this point, you had insisted on going to an amusement park. Clearly, you and Grover had two different versions of what 'fun' meant.

Eventually, you were able to persuade Grover into telling him how fun amusement parks were and that it would be a blast. Now, here you two were, sneaking into an amusement park.