There were several:
1) Pompeii - just walking down the streets after our time with the guide, saying "Imagine coming out of this house and going to the bakers next door", now that we knew that's exactly what they were.
2) Arriving at Vesuvius after horrendous rain to find the clouds had cleared and we had the full view of both the bay and the crater when we reached the top!
3) Doing part of the "Walk of the Gods" and having lunch along the way to the sound of a herd of goats nearby
4) The view from the top at Ana Capri - simply stunning!
Make sure you have cash rather than thinking you will be able to rely on cards; the only time we paid by card was the final bill at the hotel but we did end up having to find bank ATMs several times. Several places specifically asked for cash when cards were produced, despite having the usual card signs displayed. All the families in our group had the same experience. You will need more cash than you might imagine, for example our Capri day cost our family well over 100 Euros and we didn't buy anything in the shops or go on a boat trip.
If you go at the end of the season, as we did, be well prepared for rain!
If you get the chance to go to Herculaneum, do go - we weren't able to as the weather on our free day was horrendous, but the walking holiday group staying at the same time as us went when the weather was better and said it was even better than Pompeii.