It is easy to walk around El Centro at your leisure in one day. Buses run along Avenida Santander to take you to and from Laguito and Bocagrande for 1.000 pesos. Horse-drawn carriages can take you on a romantic whirl around the old city for 40.000 pesos for 45 minutes.

The only reason you would take the bus would be to visit Bocagrande, La Popa, or Castillo de San Felipe de Barajas.  Hop on a bus from Avenida Santander to Bocagrande (10 minutes) for $1.  Buses to La Popa or San Felipe (10-15 minutes) leave from India Catalina frequently for $1.

Taxis are safe and cheap.  In the evenings, get to and from Bocagrande and El Centro for $2.  Everything between El Centro, San Diego, Matuna, and Getsemaní is walkable.