If traveling within the US, a copy of birth certificate, especially for lap infants. (:
Blankets, small quiet toys, infant Tylenol, bottle brush (if you use bottles)
Portable fan, travel size baby dish soap and bottle brush or those steam sterilize bags. If you breastfeed or pump at all I found the bp wipes to be handy on the go.
Also check the airline rules to avoid spending extra on checked bags. It took me 10 months to learn that the diaper bag doesn’t always count towards your carry on and personal item allowance.
Portable sound machine if you use one, a playpen/travel crib if where you’re staying doesn’t provide, feeding covers if you breastfeed, a million burp clothes cuz you will most likely drop them on the plane, and normal baby supplies :)