The Happiness Statue in Grand Theft Auto 4 is a statue on a small island near the southern end of Algonquin, Liberty City, it's based on the Statue of Liberty in New York City. The interior of the body part of the statue is fully accessible, though the entrance is a little difficult to be found and reached.

The only way (without using a mod) to reach the entrance to the interior of the Happiness Statue is taking a helicopter.
Helicopter locations in GTA IV:
   • GTA IV helicopter locations / How to ...