At 4.5 months, babies are increasingly curious and benefit from simple, sensory-rich activities. Engage them with peek-a-boo, rolling practice, and regular tummy time to build motor skills. You can also hang colorful objects or balloons above their crib, use baby-safe mirrors, or read books aloud. Strolls outside and simple tactile play—like feeling different textures of teethers—are excellent ways to stimulate their development while keeping them entertained.