We just booked our first cruise as well, on the Valor in December. I can't wait to hear what your thoughts are when you get back.
We are also seriously considering the cave tubing in Belize. I have heard wonderful things about the tour. Hopefully others will be able to give you better first hand advice, but you may do a search and consider booking through a private tour group as well as the Carnival tour. It sounds as though both tours are comparable, but the private ones may be a bit cheaper, and the attention can be better in the typically smaller groups on the non-carnival tour.
I recently stumbled across a webpage where someone put together video/pic clips of their western carribean cruise. The couple did the cave tubing, and the zip line tour in Hounduras. It looked amazing! I will try to locate the link and send it to you.
Congrats on your first cruise, I am sure you are every bit as excited as we are.