Hi Owen,
I was there for a week during the Easter holidays, very nice island!
1 place you must go is up to Passage Peak which is about a 1&1/2 to 2 hour walk from the base of the island to the highest point of the island.
You'll see the most stunning 360 degree views. This hike is not for the faint hearted but its worth it, take plenty of water and sports bars to replace the lost fluids. And don't forget the D70!
Whitehaven beach is absolutly beautiful, unfortunatly it can also be busy so be prepared to see a lot of boats dumping off tourists. You might be lucky at this time of year as it might be a bit quieter.
There is another vantage point you can easily access with the golf buggy, from memory I'm sure its about the 3rd highest point, just keep driving up hill on the sealed surface and you cant miss it, the position next to the palm tree gives excellant views of "Crocidile Island" you see what I mean when you see it.
There is also the main harbour/marina which has some expensive looking boats to take snaps off, plus the local wildlife will keep you busy as there is thousands of cockatoo's and parakeats which are always trying to steal food from you.
The beach on the other side has some great water sports, bring a long zoom lens to catch any "action" you see on the beach!
I took my tripod and got some great long exposures at night of the sun setting from the Yaght Harbour Towers where we were staying from, if you know anyone in there it's worth trying to get some shots as each level has 360 views all the way around the building.
Any more questions just ask.