Assuming your staying along the coast...
Hat Head National Park
Its a tough one to find, but its the most amazing place I've been in Australia.
You get to the campsite, which is just your average little shady patch of grass, then you climb a hill at the edge...and BOOM
Its like you're in the Arabian desert, there are 10 storey high sand dunes every bit of the way between you and the horizon, on the far left, the beach and on the far right, the rainforest, but in the middle - desert.
You throw yourself off the top of one of those dunes and its the best fun you'll ever have, plus there's little patches of quicksand which are pretty fun to get stuck in. lol
To get there you take South West Rocks Road out of Kempsey, go past Smithtown, which at that point will reduce down to a tiny one lane road, you'll get close to a town called Kinchela at which point you take a right onto Hat Head NP Road, you follow that almost to the end, following signs to hat head camping groung, then, just before you go over a little bridge to the caravan park, you'll see an unmarked bitumen road to your right. Take that for about a km or two, then you'll see an unmarked dirt road on your right, take that one right to the end.
That road'll come out at a fork with a big sign infront of you, take the right hand fork and follow it to the end, then prepare to be amazed =]
Other than that though, the Gold Coast, Nimbin etc...