From just above the ruins you can see it all: the former senate building, the gate to Rome, and even the Colosseum. The best way to see the area is by walking around, and I would suggest hiring a tour guide. But here is a quick caveat: beware when hiring tour guides, as many are ILLEGAL! My friends and I experienced many of these illegal guides trying to sell us their services.
Also, you will want to wear comfortable walking shoes. I had come directly from the Vatican and was in my dress shoes. That made for a long walk around the site. Furthermore, it is my experience that a half-day will do at these sites. As interesting as they are, if you devote an entire day, you will likely get bored.