I went to Pompeii roughly 4 years ago. It is definitely worth the cost of admission. As everyone (including the tours) say, BRING LOTS OF WATER. As you’re walking ancient ruins, there is very little to no shade on the streets.
If you can manage it, get into some of the building which are open to the public and look around. The preserved frescoes are awesome and you get the feeling of being a Roman on the streets—down to the ruts in the streets from the carts.
Big photo opportunity! When you get to the center of the city at the forum, stand at the far end toward the old temple. You get a beautiful symmetrical shot of the columns from the forum, the altar of the temple and then Mt. Vesuvius in the background.
If you’re into Ancient Rome or love wandering ruins aimlessly, another trip when you’re in The Eternal City is a day trip out to Ostia Antica. The city is preserved pretty well (some places more than others). The great advantage is that it has MUCH fewer tourists. You’ll want to watch out for the more secluded areas of the site, but definitely check out one of Rome’s less rich port cities. There are apartments, a theatre and a preserved marketplace. Definitely also worth the trip.