We highly recommend finding a ticket/tour that includes both the Colosseum and the Roman Forum.
John and I personally thought this was just as interesting as the Colosseum. Make sure you wear your walking shoes!
Try to get here early in the morning to beat the tourist rush.
Try to purchase tickets ahead of time to avoid the lines.
If you like to shop, this is the place for you! The area around the Spanish Steps is Rome's most exclusive shopping district, with plenty of top designer brands.
Prepare to spend a few hours here if you want to experience the museum, Sistine Chapel, and St. Peter's Basilica. (Be sure to take note of the dress code.)