Cairns is one of the cities from which you can visit the reef. I loved Cairns. The laid-back atmosphere, holiday vibe, all the shops were open till 11 or 12 midnight, and the centre is pretty small and walkable. Most of the bars and restaurants are clustered around the esplanade, where all the action is.
Scuba diving - We went on the Silverswift boat ( www.cairnsdiveadventures.com.au). They have a huge double decker catamaran that goes to the outer reef on day trips. I did 4 dives over 2 days. This was something I had been waiting a decade to do, but to be honest, the diving wasn't as good as other places I have been. There were not as many fish, and I didn't see anything special other than the usual suspects. I was hoping for a turtle, but nothing came along. Maybe it was just bad luck. Also, the coral reefs are bleaching and dying due to global warming and maybe this is the reason for less marine life. I recently watched the documentary Chasing Coral and it was quite devastating for someone who loves the ocean. You can watch it on Netflix here.