Though for price and scenery reasons, we'd rather take a bus, the bus from Cartagena to Bogota takes roughly 20 hours!!
We checked several airlines and Avianca turned out to have the best price on the days we were trying to go. The final price was COP 486,000 for the two of us... about $142 USD each (one way!!)
We don't have a real firm plan yet, but we think we'll stay a few days in Bogotoa, and then take the bus down to a very small place out in the country called San Augustin. San Augustin has some ruins that Dave wants to see. From there we'll probably bus back to a town called Popayan, and fly back to Cartagena from there.
Colombia is considerably bigger than Guatemala was, and the bus rides between the big cities are very long. Even the bus from Bogota to Cali is 12 hours.
We noticed that the U.S. Embassy is having a 'flag raising' ceremony at 7:30am on July 4th. We will be in Bogota then and can attend if we want. It might be fun just to meet the Americans who call Colombia home. (But Dave's initial reaction was "7:30am!!!!")