My husband and I, as well as several guests over the past year, enjoyed these special shows/events put on in the Forum area. They are called Viaggio nei Fori and are very well produced. One allows you to walk through Caesar's Forum, the other is a multi-media presentation that one views from bleachers at Augustus' Forum. You can do both on the same evening, or split it up over two nights.
I don't know that you have to buy your tickets terribly far in advance, but maybe a couple of days is a good idea in peak season. HIGHLY recommend and a wonderful thing to do at night during the hot weather.