It's a lot to look out for. The trick is not to make it feel like an exam. School's out, the last thing they want is a lecture or a lesson. Instead, give it another function. Ask them to run you down to the shops to pick up tea or to drop you off when you're heading out for the evening. Use this time to watch how they drive and offer them friendly pointers.
The more driving you and your teen can experience together the better prepared they'll be. Keep an eye out for opportunities. For instance, is your family off on holiday this summer? If so, why not share the driving? It's a good chance for your teen to get used to spending long stretches of time on the open road, while you're there to supervise them.
As the day of your teen's long solo drive approaches, focus on preparation. Talk with them about the dangers of driving tired and distractions like loud music, gadgets and food. Go over their route and point out potential rest stops and places to grab a snack. If your teen is well rested, they're less likely to make bad driving decisions. Finally, remind them about the obligations of their restricted license: no driving between the hours of 10pm and 5am, no passengers* and no alcohol.
With your preparation done, there's nothing left to do but kick back and enjoy your summer break.
*(with some exception see nzta.govt.nz)