Guinea pigs express happiness in various ways, including popcorning, which is when they jump and twist in the air, making a popping sound. They also purr or whistle softly, chirp when they are excited, and groom themselves or their cage mates. Additionally, a happy guinea pig may wag its tail or rumble softly. Providing a comfortable environment, regular social interaction, and a balanced diet are essential for keeping guinea pigs happy and healthy.