There are many types of happy games for babies aged 3-6 months. Here are some suggestions:

Ring: You can give your baby a soft ring to shake and hear the sound, increasing their auditory sensation and hand coordination ability.

Clap game: clap hands with your baby, teach them to clap hands according to your rhythm, and improve their visual and auditory coordination abilities.

Toys on the Game Blanket: Placing toys suitable for the baby's age on the game blanket allows the baby to grasp, touch, and explore on their own, exercising their visual and tactile abilities.

Parent child reading: Choosing picture books that are suitable for the baby's age and reading them together can help the baby understand the world and improve their language ability.

Gently rocking: You can give your baby a gently rocking bed, or gently shake your baby's arms, allowing them to experience different levels of gravity and balance, enhancing their body's sensory abilities.

In short, babies aged 3-6 months need simple, easy, and fun games that can help them improve their senses and motor abilities. Meanwhile, parents should also pay attention to safety to ensure that their babies do not get injured while playing.