The most important check you can perform is your coolant, if the coolant level in your cooling system is too low you run the risk of your car overheating. Nowadays, most cars will let you know if the coolant is low on the dashboard, however, if this is something you need to do manually, the AA have an article that gives you step by step guide to your vehicles coolant: Checking Your Engines Coolant.
In the hot weather, be aware your tyre pressure is likely to increase as your tyres overheat and inflate. Todays tyres are designed to have a high heat resistance, but during a heatwave, you should be checking your tyre pressure at least every week to ensure car safety.
There's nothing worse than air conditioning faults in the summer. A full service will include a full air conditioning check for you to find out the current condition of your air-con, but there are also things you can do yourself. Bell Performance have written an article on what you can do, and what you need to leave to the professionals: How To Maintain Your Car's Air Conditioner.
ON BOARD BAG
When going on a longer journey, we always advise you prepare for the worst in the summer months and winter months. Jumper cables, a water bottle, sunglasses and a windshield screen are things we would consider essential for summer driving. You may also want to bring things like sun cream, a hat or a map if you need to leave the vehicle for a walk, although these things aren't as important for safe driving as those above, they are still a good idea for a comfortable road trip.