A few suggestions...
if they didnt have it on their registry, I always get people the Kiddopotamus "Snuzzler" and everyone I know loves it. Its amazing how many people dont register for it. Its perfect for in the carseat as opposed to the head rests, works in strollers, swings, baby joggers, etc. You can get it at babies r us, meijers,(i dont think target carries it except online)
the other thing someone got me was a little plastic caddy with infant tylenol, motrin, infant qtips, cotton balls, vaseline, etc etc....you could pick what you want to put in it...she tied up each individual item with blue and white tissue, put them in the little tupperware caddy, and gave it to me that way. even if they have those things, they will use them over and over!
if you are looking for something less expensive, I often get new moms the "snack trap" cups...these are great, and most new moms dont know about them. Granted, its for when their baby is a little older and able to eat finger food like cheerios, etc, but my four year old still uses them in the car for goldfish, etc. they have these at babies r us, toys r us....and they are called Snack Trap.
lastly, its always great to get someone a few books for the baby. i usually go to tjmaxx where they have a great selection of board books for babies, and are far less expensive.