If you know your child is prone to motion sickness, consult your pediatrician well in advance to find out any measures you can take to lessen it. A sick child will make for an extremely uncomfortable trip for all.
Essentials
Besides the items already listed, make sure to pack all medications that your toddler may require, including any ongoing medications such as inhalers. Do not rely on a pharmacy in a strange place unless you have to. Make up your own kit with items such as Band-Aids, tweezers, disinfectant, sun blocker, and other sun aids including a hat, sunglasses for warm weather, and sunglasses and warm hat for a snow vacation, lip balm, and Tylenol or Motrin, depending on your preference. Also, include bug bite soother, child safe bug repellant, lotion for dry skin, (pool water, altitude, etc., may dry your toddler's skin more than normal) etc.
Parent's Bag
What you include in your own luggage can also be the difference between a successful trip and a disaster. Besides your basic luggage, including your clothing and gear specific to the trip, make sure to pack a cell phone, change, or calling card for pay phones for those rare times when your cell phone may not work, and a purse or wallet that can be strapped to your body. Never depend on a regular purse or wallet when traveling with a toddler in tow. It is too easy to set down when you suddenly have to set chase to a rogue toddler. By the time you realize it, your purse or wallet may be long gone, and that is not something you want to be dealing with whether you have children with you or not. If you depend on glasses to see, pack an extra pair or at the least, alternate wear such as contact lenses. Keep in mind when traveling with a toddler, everything that you take for granted can be compromised if lost or broken, and running to the eye doctor may be easy when at home, but can be a major disturbance when traveling.