alright i just got back from Cairns...its a GREAT place..there are cute girls and lotsa stuff to do (well actually not THAT much)
a 7-day trip would probably be enough for you to do just about most of the attractions there
the Whitewater rafting in the barron river may seem hard, especially with the words "grade 3 rapids" on the brochure (for people who have never rafted it seems quite big lol) but seriously the rapids were not really that amazingly intense or hardcore. I'd say that swimming in the river towards the end would have been the most interesting overall...for those who are in for adredelin rushes whitewater rafting on the barron river aint that good a choice and i'd say u take the tully river instead, which apparently is much more..."hardcore"
theres skydiving to do too (alot of other places have skydiving but i thought i might just mention it). i personally did not take it but apparently its not that great a thrill (unless u fear heights i guess)
basicly the big main super attraction of going to Cairns would be the Great Barrier Reef. A one day trip there is really good, theres snorkelling in a safe enclosed area of the reef where u can swim freely around and explore the reef about you for a certain time (much like a swimming pool with closing times). Theres also scuba diving available, for first timers and for experienced, certified divers. if you are in for fun and a great experience i'd really really recommend this, the great barrier reef may be impressive while snorkelling around with limited view, but scuba diving to the bottom and experiencing the reef close up is breathtaking.
there are hot air balloon rides to take too...which is actually a matter of praying for good weather to have a nice flight..my flight was not really high as the wind that morning was not really favourable...we only went to an altitude of over 1000 feet at best...not even close the 4000 feet limit...then theres the packing of the balloon which is quite interesting...sadly enough some guy lost his digi-cam in the process...
basicly Cairns is a great place to go, for a short holiday or for a place to kick back and spend the rest of your life (preferably when ur loaded).