One of the pioneers of cave-tubing in Belize, Vitalino Reyes is still a reliable and recommended operator. There are a variety of cave-tubing trips (including equipment and lunch), up to the full-day Crystal Cave (BZ$180) and 'sunset tubing' (last entry is at 4pm). Vital also has a zipline course and ATVs (quad bikes).
A combo with all three activities is BZ$200 per person. Add BZ$40 per person for transfers from Belize City or Belmopan.