Do not plan a vacation with a toddler at the age of 3-9 months old. Take the trip while your toddler is sleeping and enjoy the trip because it will be a very long trip. If your baby is drinking formula, half of that vacation just won't exist. If you are taking your toddler out with you, remove all of the predictability from your trip. You may be expecting him or her to sleep the whole trip. That just doesn't happen.
A toddler will not fall asleep. Make your vacation as relaxed as possible. Make sure the other children or adults are relaxed as well. This will not be a vacation. In fact, it might be more like you are on an airplane. Sit in the middle of the cabin and while that is not possible, entertain your toddler by reading him or her a book, singing songs, going out for ice creams or whatever you choose to do. Even just an hour of your undivided attention will really make a difference.
Pack snacks that you like to munch on throughout the day.
Don't pack a bag for one meal and expect to eat everything you have prepared. That just doesn't work. Pack one bag for snacks that you really like.
Plan activity activities during your vacation to enjoy with you toddler.
Instead of bringing formula, pack a few of your child's favorite snacks, so you are not trapped lugging a bag full of processed, sugary treats. You are going on a vacation. You don't need to carry a gas can where everyone climbs in and then says "I'm hungry" and can't leave the room.
Take a jigsaw puzzle with you, if you have one.
Lastly, bring a portable computer. This should be a portable laptop or a Kindle powered book. You won't realize you are on vacation if you pack a word document into your purse and take it with you. The key to make planning a vacation all it can be is to plan in advance, and take these tips along with them. Be relaxed, not stressed, so that you can enjoy your vacation with your toddler!