went to mexico years ago... was a trip of a lifetime to be honest... my favourite was the yucatan peninsula, but im a sucker for jungles and temple ruins... oaxaca is quite beautiful too, but dress appropriately as it can get quite chilly even in the height of summer. the pacific coast if beutiful and really hard to find a miserable place... went to some natural parks near the southern border which were fantastic as well, beautiful clear blue lakes in old volcano craters... mexico city will also fill up a whole week in no time for sure...
but whatever you do do not miss out on the fantastic food they have there.