With a new baby will come lots of free things. Take them all. You will either find a use for the items, or will know of someone you can give them to who will have use for them.
Some advice. Get a big diaper bag in a simple dark color. The dark color will show the dirt less. Always pack two outfit changes minimum. I am still amazed by the amount of poop a baby can produce, and the force with which it is expelled from their bodies. Remember also that, in the beginning, it will be almost all liquid, and diapers are not watertight. Hence, the need for additional outfits.
Pack as many diapers as you think you will need, and add two more. The darned tabs rip off sometimes, and my sons also had a bad habit of relieving themselves in the middle fo a change.
Baby wipes are essential. Pack more than you think you might ever need. Those cute little containers are nice, but don't hold as much as you will need. Take two or three. Keep them full.
Always keep a current bottle of Tylenol or Advil in the bag, especially during teething.
Shout wipes or a Tide stain stick. I still remember holding my oldest, straddling my leg as we signed up for our first cell phone (6 years ago). The warm wet feeling on my leg, followed by the laughter of my wife and the saleswoman, and the sight of the growing borwn stain on my khakis...you get the picture.
Love every minute of it, because they really do grow too quickly.
