I may not be a practicing archaeologist anymore, but the ancient world is always very much in my thoughts every day. Thousands of years between then and now, and thousands of miles, but we Westerners are still very much creatures of ancient Rome and Greece.
What I have found is usually the hardest for people to understand outside of the abstract is just how big the city of Rome really was. It had over a million people in it at its height, and had to provide all the city services that come with making a livable city for that many people: water, sewer, food, markets, commerce, entertainment… This video does an amazing job of getting the point across, while giving some delicious eye candy. Check out that aqueduct! And keep in mind the size of some of those things, especially when you see them right next to the Coliseum.