Scuba diving is fun, scuba diving on the Great Barrier Reef doubly so (or at least that's my estimate, because this was my first time). I recommend it!
We found Nemo, various other fishes, corals, giant mollusks, and supposedly a shark hiding in a reef which the instructor tried to poke at, but it didn't want come out. We got to pet a couple different kinds of sea cucumbers.
I also tried snorkeling for the first time, but found it more difficult than diving. I kept getting water in my snorkel when I tried to dive with it.