Por vs. Para can, indeed, be a complicated subject.  However, in my classes (beginner to intermediate) my focus was on the quickest way to communication and I decided to forgo the three pages of instruction found in our textbook.  I condensed por vs. para into three rules!  These are English-speaking children and they had no problem with "por" when it meant:  "by, through, along, per.".  The confusion came with the meaning "for" which both "por" and "para" translate in English. 

Use POR to express:

a.   In exchange for                    (Te doy $10 por el libro.)

b.   On behalf of                         (Enrique no pudo hacerlo, así que lo hice por él.)

c.   A period of time                  (Estudiamos por 3 horas.)

Use PARA for ALL other meanings of "for".  Examples:

For an old man, Mr. García is very active.        Para viejo, Sr. García es muy activo.

I made this gift for you.                                      Yo fabriqué este regalito para ti.

Salimos para Granada a las 9:00.                    We left for Granada at 9:00.

This doesn't cover ALL the uses of "por" and "para" and is somewhat short on specifics.  However, it covers (in my estimation) 80% to 90% of everyday communication needs and doesn't overwhelm beginning students.

Clara

Como siempre, muy interesante. En francés, normalmente por=par y para=pour; pero, evidentemente hay ‘excepciones’. Cuatro inversiones en la lista.

¡Aquí son las trampas!

Gracias por sus ejemplos.