Pompeii was a large town, with 15,000 residents, so there is a lot to see when walking around the ruins. Even to this day they're still excavating undiscovered features of the town.
Be sure to pay a visit to Pompeii's large theater, numerous temples and the markets. For a more unusual experience, head to the brothels (Pompeii has more brothels than temples!). In the wall of each room you’ll find a painting of a different sex act. The residents of Pompeii, business minded as they were, put these up for the aid of foreign travelers who had docked in the port for the night, so that they could just point at what they wanted without negotiating a language barrier.
Many of the tours offered also provide a trip to the crater of Mt Vesuvius, which most recently erupted during a bombing campaign in World War II, so make sure you pick the right one to make the most of your journey.