I second the cool mist suggestion. I think we use one that's evaporative. I could be wrong but I think the ultrasonic ones spread more mineral dust if you don't use distilled water. Either way, both should get sanitized every now and then. The evaporative ones might be louder, which could be a benefit if you're looking for white noise.
Also, in general, I remember reading cool mist is better in a warm climate and warm mist in a cold climate. But, definitely for a baby room, they recommend the cool mist.