If you purchased the three-day ticket for Pompeii and Herculaneum, the villas at Stabia are included and not to be missed. They are quite a way up hill, one at a higher level and therefore further than the other. We had a hire car and visited on different days but if you are on foot or using public transport, you will want to see both villas on the same trip. The villas (Villa Arianna & Villa San Marco) were luxurious properties for high-status Romans, so they give a somewhat different perspective to what you will see at Pompeii and Herculaneum. The full extent of Villa Arianna may never be known as it has not been fully excavated, plus some rooms have fallen down the cliff. Arianna has a small visitor car park. Villa San Marco is more complete. A car park for San Marco is clearly signed. Don't then be put off by a further sign which is a little ambiguous because the walk along a path into what looks like private property may seem unlikely, but this is the way. You will also find that there are many fewer people taking the trouble to visit these sites than you will see at Pompeii and Herculaneum so you will also escape the crowds. At Arianna at the end of the day, we were the only visitors at the time.