Id suggest taking a night tirp to red fern in sydney with an expensive video camera (one that will get really good pictures and video) and trying to catch a glimpse of the rare australian aboriginal. Make sure to walk down ever dim light alley way becuase that is where the usually hide.
Other things I suggest, go to bondi and surf, go to manley and surf, go to the blue mountains for a trek. take a jet boat ride in the sydney harbour. walk to the top of the sydney harbour bridge. Dee Why, Narrabeen, Cronulla also are sick breaks and fun places to surf. Oh and the sydney aquarium is pretty cool as well. If ya got three weeks though, rent a car and jet up the east coast. sydney only takes a 3 or 4 days to see it all. Check out Oz experience when you get there or just look it up on google. They offer really good 3 day to week long hiking packages into the mountains. Hit up byron bay, collongatta and other surf towns up the east coast. oh and dont forget to take your parents for a day trip to Nimbin. great town, great culture.
If you really want a a quick and easy way to see it all. Stay in sydney and area for a half a week and then get on a plane and fly to brisbane. tickets are like 70 bucks. then rent a car, hit up surfers, byron, the water parks, and if you want a sweet place to relax, check out noosa heads. I love that place. If you want to do alot of adventurous stuff. check out peterpans when you get to oz. they offer alot of packages to go hang gliding over byron, white water rafting, sailing trips, 4X4ing on fraiser island. scuba or whatever. plus they clump it all together for you with dates and stuff so you can make sure you can do it all over your three weeks. My buddies did the entire gold coast in three weeks and saw it all. I spent 10 months but I lived most places.
