Opportunities for day trips are available, depending how far you want to go. The combie (a microbus) goes as far as Petatlan if you are heading North, or Papanoa if you go South. From there the mode of transportation vary immensely. If you would like to continue North past Petatlan, you get on a larger bus, comparable to a school bus. South, in the back of a truck. North is the more popular route towards Zihuatanejo.
Between Petatlan and Zihua is a fairly new archaeological site called La Chole. http://soledaddemaciel.com/petatlan.htm
Trip adviser has many positive comments on this site as well