Ixtapa-Zihuatanejo is cheap and it is really for relax. If you want all inclusive, Presidente Intercontinental is a good hotel.
You can try Best Western Posada Real, it is not all inclusive but there are a lot of cheap restaurants around and they have low rates, besides they have the BEST swimming pool in whole Ixtapa.
You can find more information here:
http://www.ixtapa-zihuatanejo.com/info/english.htm