I highly recommend Charlie's Cave Tubing. Highlight of our whole cruise. We went tubing in February last year and it was very relaxing just floating through the water. Zero work on our part. The tour was private (very reasonably priced) and we added on a longer hike for a small extra fee per person. As a result we walked much further than all the other cruise guests and saw much more flora and fauna including howler monkeys, then tubed through two extra caves. We had those first caves all to ourselves before getting back to where the other cruisers were. We had three of us being paddled by our one guide, where other tours had very long chains of people.
Check out a video of our experience here:
YouTube - Cave Tubing in Belize
Have a great trip!