This stimulation idea incorporates touch, vision and tracking, and cause and effect: with your baby lying on his back hold up a large pompon. Say, "here comes the pompon" then let it drop onto baby's belly. You can then tickle his belly and then give him a kiss. Your baby will not only be stimulated, but very happy to be interacting with you!
And be sure baby gets a chance to check out the pompon. You wouldn't want someone dropping something on your belly unless you knew what it was!