Check/Top Off Antifreeze
I've passed many a person, pulled off the road, pouring water in their overheated vehicle. Let's avoid that, shall we? Take a peek under your hood and check your engine coolant level. If it's between LOW and FULL, you're good to go. If it's below LOW, have a professional check for leaks.
Check Battery for Leaks & Cracks
Make sure the box powering your car is damage-free. If you find corrosion, it's leaking acid and will need to be replaced. Bite the bullet. It's easier to buy a new battery than drive with vinegar and baking soda to clean the terminals when your car won't start. (Yes, my sister totally did that.)
Top Off Wiper Fluid
Driving through a bug-infested stretch or a dust storm is pretty par for the course on my road trips. But if I'm out of wiper fluid, an annoyance has become a hazard. You have to see to drive, so fill it up and you won't have to stop at a gas station just to clean your windshield.
Once that wiper fluid is filled, run it through and check your wipers at the same time. Again, visibility is key and you don't want to be caught in a rainstorm with a streaky windshield. Replace those wipers, even if the forecast doesn't call for rain.
Have a family member or friend stand outside while you hit the breaks, turn on headlights, and signal for turns. I've been pulled over a few times not knowing I had a taillight or headlamp out. Don't risk getting a ticket on vacation!
Check Tire Pressure & Tread
Before you fill up with gas, take advantage of the station's air compressor and check the air. Look at the sticker on your door frame for the recommended tire pressure. For tread, use the "coin trick." Stick a penny in the tread of your tires. If the tread covers any part of Lincoln's head, there's still life in your tires. If the tread doesn't touch his head, it's time to replace them.