Your willingness to slow down and enjoy the trip. Don't leave your ability to slow down and smell the roses at home, or your ability to just sit and enjoy watching your little one spend 20 minutes watching a baby duck swim and walk around instead of rushing to the next ride or character.
The most common things I forget are Suntan Lotion, sippy cups, and activities to keep the little one busy while waiting (plane trip, in line, etc.). My sunglasses are also something I often forget, for they normally stay in the car.