You can take a train to get to Pompeii from Rome (2 hours) or elsewhere. You can also book a day trip that includes a scenic drive along the Amalfi Coast. You may reach the top of Mount Vesuvius by car or with one of the many bus services from Herculaneum or Pompeii.
Colosseum
The Coliseum can be easily reached by public transport or by foot from anywhere in central Rome.