Not quite like Sandals, but Kanantik between Placencia and Hopkins is a true all-inclusive, as is Coco Plum Caye off Dangriga and Mopan River Resort in Cayo (Benque). They all offer packages that include lodging, meals, drinks, some or all activities and tours for a per person price, and Mopan River, really the first true all-inclusive in Belize, also throws in tips, taxes and transfers.
Cayo Espanto on the back side of Ambergris is also all-inclusive, though at a price.
There are a few other places that will do all-inclusive type packages, but as suggested Belize is not an all-inclusive type of destination like Jamaica or Playa del Carmen.