Walking through the ancient city of Pompeii is an excellent living history lesson. The site is extensive and you can easily fill a whole day exploring. You can walk the streets, and visit ancient theatres, baths, shops and houses - all with Mount Vesuvius as a backdrop.
A must-see, although with children we would recommend keeping it to just a day; a full day of temple touring is pretty tiring but is enough to see all of the main sites, plus a few more. If you insist on seeing more, it wouldn't hurt to have a day's rest in the middle.
We stayed at, and highly recommend, the King Boutique Hotel. It has a pool (it gets very hot there), is fairly central, so it's easy walking distance to restaurants, and is run by a lovely family. They arrange free pick up from the bus station and can also arrange tuk-tuks for your visit to the temples.
Always travel with the Giant Ibis bus company in Cambodia if you possibly can. Their service is excellent, the buses are new and clean and safety is a high priority.