1. Battery
The summer heat can certainly affect your battery, in fact heat is one of the worst things for a battery. When you are driving your vehicle for those long summer trips, the extra vibration coupled with the heat can really damage your battery. It would be wise to bring jumper cables or a battery box with you and be sure to check your battery for corrosion.
2. Cooling System/Coolant
In the summer months coolant is one of the factors to keep in mind. Low coolant can kill your engine. This is more common in the summer months because your vehicle is working extra hard to keep the right temperatures and can fail due to being overheated. It is important to pay attention to your gauges to try and prevent overheating. You will also want to make sure your coolant tank is full.