First, the hotel chain has partnered with Hawaiian Airlines offering $50-100 off flights from the mainland to the islands this summer and fall! See https://apps.hawaiianairlines.com/Affiliate/Booking.aspx for details.
Second, whether you stay at these Honolulu properties or not, just show your membership card to receive discounts on food:
Kai Market (Sheraton Waikiki) - 15% off locally grown food and organic dishes in an outdoor patio atmosphere.
Rum Fire (Sheraton Waikiki) - 15% off dishes and speciality cocktails including rum based drinks.
Side Note: SPG points expire every 18 months so be sure to visit the Waikiki Sheraton or the Moana Surfrider (or their bars and restaurants) every now and then to keep your (free) membership active on Oahu.