I highly recommend this toy for any baby. It attaches to the crib, and offers soothing music and sounds, along with pretty lights, bubbles, and little things the baby can play with to make the clam go up and down, spin the sea star, etc.
We bought this for my eldest, when she was around 5 months old. She played with it constantly, and I honestly couldn't get her to go to bed without the soothing music it plays, on some nights.
When her little sister was born, we brought it out again, and she loved it just as much. It was their own personal little aquarium!
This is an "R exclusive" so you'll only be able to find it (new, that is) at either toys r us or babies r us. It's inexpensive, and you won't regret your purchase!