During the Planning Stage
These link are helpful while you're planning your vacation

Apply For or Renew Your Passport - Sometimes getting your Passport or even renewing it can take months. It's best to get this out of the way early. If your Passport expires within 6 months of the end of your vacation, you need to renew your passport to avoid any travel problems.

Global Entry - This program helps smooth the trip through immigration and customs during an international trip.

TSA PreCheck - This program helps speed the process of going through airport security. With this program, you don't have to remove shoes, belts, or computers. It really speeds up the process.

Travel Advisories - Check travel advisories of the places you are thinking of traveling to.

Before Your Vacation
These links are helpful before you leave for your vacation.

CDC Health Warnings - Check the latest health warnings and travel info from the CDC.

Weather - Check the weather for each stop along your way.

The MPC Mobile App  - The Mobile Passport Control allows U.S. citizens and Canadian passport holders (on B1 or B2 visas) to submit their passport information and customs declaration information via their smartphone or tablet and bypass the regular line to enter the United States. The app doesn't require pre-approval and is free to use.  

TSA Guidelines - Keep up-to-date on all the guidelines from the TSA.

Cruise Port Schedule - If you're going on a cruise, check which other ships will be in the ports you are visiting. This will give you some idea how busy the port will be.