I went to Cairns for 3 days/4 nights at the end of September as part of a larger trip across eastern Australia.
I second the motion for TUSA Dive. My avatar photo was taken on one of my dives with them. While they might not appeal to the hard-core diver, they were perfect for an inexperienced diver like me. The boats were never crowded, and the DM-diver ratio was about 1 to 4. They serve a good lunch as well, and if you are staying in Cairns they will pick you up from your hotel; I don't know how far outside of town they will go.
There is plenty to do in the greater Cairns area other than diving. Unfortunately I didn't have the time to do this, but I would have loved to go up to Kuranda on the historic train and returned via SkyRail.
If you want to go for a nice leisurely drive, there is a nice "loop" drive that starts/ends in the north end of Cairns, goes up to the "Atherton Tablelands" which is a mesa overlooking the Cairns coast, and finally ends/starts south of Cairns. I'm not sure if you can rent a motorcycle in Cairns, but if you can and have a motorcycle license, the roads leading to/from the Tabelands are excellent for motorcycle riding.
http://www.athertontableland.com/
You do NOT need a 4wd to go to Townsville unless you're planning some weird route. Highway 1 is paved all the way to Sydney and beyond. Once you leave Cairns proper though, it becomes a two lane road with occasional passing areas. In my experience, Queenslanders tend to drive about 20 km/hr below the posted limit, so give yourself plenty of time. This road also has a pretty high accident rate, if the numerous driving safety billboards are to be believed, so be careful when trying to pass. Queensland has numerous "Driver Reviver" sites along this highway where you can pull over, take a nap, use the restroom if necessary, and unless it's the early morning hours there is a volunteer there serving free coffee. Take advantage of them; they recommend a 15 minute break after every 2 hours of driving.