The only thing MiraMundo does not provide is your international flight. Everything else from the moment you land is organized. You will be picked up at the airport and dropped off at the end.
The biggest airports in Colombia are Bogotá, Medellin and Cartagena. Most people will start their trip in Bogotá or Medellin and end in Cartagena.
On most websites you can specify multiple destinations, so that you can fly to one destination and back from another destination.
Some websites where you can book airline tickets: