One thing you can compliment is the expressions the baby makes. Maybe the baby is especially bubbly or maybe they look very serious and thoughtful. Either way, use that information to compliment the baby. For instance, you could say, "Look at that smile! She's just beaming." Even if a baby isn't particularly adorable, most babies have something cute about them. It could be their ruddy cheeks, their chunky thighs, or even just the puffed out belly. Find something cute to compliment. For instance, you might say, "Look at those adorable rosy cheeks!! I could just eat them up." Another tactic to avoid discussing the baby's looks is just to offer up something nice about the baby's outfit. As a bonus, you're also complimenting whoever picked the outfit out. Try something like, "That outfit is just precious! Did you pick it out?" Another aspect of the baby you could compliment is how happy the or quiet the baby is. Often, that's seen as something praiseworthy, so most of the time, the parents will enjoy this type of compliment. Say, "Oh, he's just such a happy baby! Always laughing!"

Summary:
Compliment the baby's expression. Pick something cute about the baby. Compliment the baby's outfit. Say something nice about the baby's temperament.