The oldest, and one of the most distinctive national parks in Australia.
Although relatively small, the park covers a fair range of geography and vegetation, with some fairly remote and untouched forest a bit further in from the coast and the three famous tourist spots. Unfortunately a major fire last January has made quite a bit of the bushland areas of the park blackened and unsightly, but Australian bush bounces back quickly from that sort of thing. Close to the ocean, however, the park is pristine and spectacular.
To the north, closer to the city, the sea cliffs are lower, with more exposed soft sandstone, often weathered into beautiful rounded white shapes. The vegetation here is mostly flowering heath along the the ridges, with quite a lot of exposed stone and a few small eucalypts. There are some areas of swamp, a habitat for frogs and more than a few snakes, and patches of forest along the shallow, sandy creeks. There are some very good walks along the cliff tops from Bundeena or Wottamolla over this kind of terrain, with spectacular views of the ocean. For a few months of the year, migrating whales are increasingly a common sight, and the cliffs are an popular spot to sit and watch the pods swim past close to the coast.
The popular Wottamolla area is about halfway down the park. The beach is very safe, on a long, sheltered, shallow bay with very little surf - the bay once provided shelter to the explorers Bass and Flinders during a severe storm. The beach and freshwater lagoon are popular with families with small children, but there is also some good snorkelling to be found out near the entrance of the bay. There is also a wonderful short day walk along south along the cliff tops that starts at the top of the Wottamolla carpark.
South of Wottamolla, there is one small beautiful bay with fantastic snorkelling, and then the land gets higher. For several kilometers, the coast is entirely made up of huge sea cliffs. This continues to the northern end of Garie Beach, one of Sydney's better surf beaches. I have to disagree with ymelup, I have been going to Garie for many years and there are lifeguards on duty every weekend in all but the worst weather. The beach, however, is certainly dangerous and swimming is only recommended if you're experienced in serious surf.
South of Garie, the cliffs are replaced by a long, steep escarpment with small cliffs and beaches at the bottom. The vegetation gets progressively thicker, with dense forest and even some areas of semi-rainforest. The Burning Palms area is unfortunately marred by a tangle of semi-legal squatter's huts and a surf club, all accessible only by foot, down a muddy path that always smells of sewage. Once you get past this, however, the beach is nice. It can also be approached from the southern end, down a long but dramatic walk through dense palm forest, opening into woodland higher up. At the top of the escarpent, an hour or so walk from the beach, there are some large flat rock platforms overlooking a densely forested valley and inaccessible beach. The platforms are occassionally used as a launch point for hang gliders, and the view is spectacular.
Similar terrain continues south along the coast to Oatley, the end of the park. Inland, there are large areas of forest, some rainforest, and a couple of decent sized rivers. The southern end is much hillier and densely forested, more inaccessible and with fewer walking trails than the coastal areas further north. The park does not extend very far inland, however: its western boundary is the main coastal railway line and freeway, along with a few small towns. The bushland continues on the other side of this strip of civilisation in the Heathcoate artillery range.
The main access road through the park is Lady Carrington Drive, which is incidentally a very fun road if you have a good car, particularly on weekdays when there is less traffic. Entry to the park is $10.
Anyone interested in more detail of walks, beaches, park access and so forth can drop me a message any time and I would be more than happy to provide information. It's a great area.