Hey, look who's looking! Babies love to check themselves out in a mirror. In fact, faces are your baby's favorite things in the world to see at this age. Hold your baby in front of a mirror and watch her delight. She won't understand that it's her own reflection until she's about 7 or 8 months old, but she'll enjoy the view. And the motion of her hands in the mirror will capture her attention.
Babies who are 3 and 4 months old also enjoy being shown books with big baby faces (don't expect a long attention span, but you'll get a brief blip of interest). Consider both cuddly, soft books that are interesting to feel and sturdy board books that are designed to withstand gnawing. Pictures of babies in magazines and on food packages are also likely to be big hits, so share them when you see them.

When it comes to looking at people, of course, your expressive face is number one!