I live in Cairns and have done about a dozen 3 day trips on Mike Ball to the Ribbon reefs. The ribbon reefs (80 KM north of Cairns) are far superior to the reefs directly off cairns. Thousands of people visit the reefs directly off Cairns every day. No day boats go to the ribbon reef dive sites that the liveaboard boats go to on the ribbon reefs so you will lucky if you see another boat in the distance and the only 2 sites you may get 2 liveaboard boats at a night dive site called Challenger Bay (Ribbon Reef #9) and possibly at the Cod Hole (top of ribbon reef #10). Both sites are large so there is an opportunity to spread out.
Density and variety of fish and marine life is a way to describe the ribbons. Just about every where you look you will see stuff. There are many great sites (Steves Bommie, Lighthouse bommie, Pixie Pinacle, Cod Hole, Flare Point, Vertical Gardens (weather dependant), Snake pit - when the conditions are good this is a really interesting site. Howevery my favourite of all is a site called Acropolis - they only started to dive it a few years ago and the life on the site is amazing plus it has some of the best hard coral formations you will see anywhere. (big call but i think it is justified) As for marine life - fantasitic - videoed a 350 kg Qld Grouper last time i dived it (on my facebook page 1 min 20 secs). In addition to this, the 3 day trip takes a detour out to the osprey reef system. There is a shark attraction dive (40-50 white tip and grey whaler sharks) - no hand feeding is done. You perch youself in 15mtrs of water in a natural amphithetre at the the end of oprey reef - they lower a tin full of fish frames and the sharks are attracted to this and come in and swim around and over you. You are not part of the equation and the sharks are not interested in you at all. Have also seen a great hammerhead and mantas at at dive site called "round the bend" at Osprey reef. Rhonopeas (lacey scorpion fish), mantis shrimp, cuttlefish, reef octopus,turtles (loggerhead and hawksbill), leaf scorpoin fish, olive sea snakes, napoleon/Maori/humphead wrasse, bumphed parrot fish (1.2mtrs) are some of the great things you can see on the ribbon reefs - too many to mention them all.
and if that is not enough - you could consider doing a 3 day trip between mid June and Mid July and you could swim with dwarf Minke Whales! (8 mts long baleen whale - very beautiful) - I have done is trip 3 times and it is fantastic and would really recommend it. There is a picture of a Minke whale on my face book site.
hope this helps