Check the expiration dates on your drivers' license, passport, credit cards, and any other travel documents you'll be using.  While you can sometimes get on your flight with an expired ID (emphasis on the sometimes), you will not be able to rent a car.  Yes, I'm writing from first-hand experience with this!  This can completely ruin your family vacation, especially if you are the only adult traveling.  Keep in mind that registries often do not let you know in advance when your license is about to expire.

Make sure you pay all of your bills by their due dates.  If not, you may be faced with some major late fees.  Where possible, also pay bills that are due several days after you return.  When you come back from vacation, you may not want, or have time, to pay bills right away.

Check with your cell phone provider and your bank to see if what texting, data, phone call, and ATM fees will be at your destination (assuming you are traveling oversees).

Let your credit card company know you are traveling, especially if you are going to several destinations.  Otherwise, they may start declining your card as a security precaution.  It can be a huge hassle to call the credit card company to reinstate your card, once you are on vacation.

Check with your credit card and car insurance company to see what type of car rental insurance you'll need.  Often, you are already covered and will not need to purchase supplemental insurance.  But, that's not a decision you'll want to make at the car rental counter.

What other financial things do you take care of before heading on vacation?