When you head out on a summer road trip, you're not only leaving home, you're also leaving your trusted mechanic. If you have car problems on the road, you'll have to find a mechanic and then spend hundreds or thousands of dollars (and more than a few hours). You may, however, be able to avoid all this hassle by simply preparing your car for the trip.
First of all, check all of your car's fluids including oil, coolant, transmission fluid, brake fluid, power steering fluid, and windshield washer fluid. As a general rule, each of these fluids should be more than half-full. You can do a leak test by leaving the car running for ten to fifteen minutes and monitoring the ground below the car for fluids.
It's also a good idea to go ahead and change the oil and the engine coolant. Summer driving puts stress on the engine, so you may want to use a higher viscosity oil than you normally do, and you're supposed to flush your coolant every two years regardless of how much driving you do.
Then, check the engine and the brakes, since they'll face high heat and increased usage during your trip. Do the engine belts look worn or are the hoses spongy or brittle? As for the brakes, if you're going to need to replace them soon after your trip anyway, go ahead and replace them before you leave.
Finally, since it's the summer, have your mechanic check out the air conditioning system. Nothing makes for a grumpy backseat faster than a broken air conditioner.