Japanese friends say that its a long way for nothing, not really interesting to travel so far and there is nothing there. I have a different view, it is a natural haven for Australian flora and fauna and the area is full of history. For the $12 park entry fee, you can visit West Head Lookout and you can see information about the natural landscape, photographs of the World War 2 gun emplacements, see local aboriginal rock carvings and spectacular views at the mouth of the Hawkesbury River, Lion Island, Barrenjoey Headland, Palm Beach and the seascape of Pittwater (with thousands of sailing craft on weekends).
Another slice of history to investigate is the location of a Japanese submarine that was scuttled off West Head during World Ward 2, after the highly publicised raid on Sydney Harbour in June 1942.
You need to take a picnic lunch and a thermos of hot tea or coffee and you will have everything you need to enjoy a half a day there. There are plenty of walking tracks for all levels of fitness, and so many aboriginal carvings from thousands of years ago, you will take with you a new level of expertise on these historical masterpieces.
If you visit in spring, the park is full of wildflowers. An easy place for viewing rock carvings is the tessellated pavements rock platform, just off West Head Road near the Elvina Bay car park. Find more information is Google and look up: "aboriginal carvings at west head", for the best locations. When you go the West Head lookout, the toilets and picnic areas are about 500 meters before the lookout and remember that there are no shops or cafes, so take your own food and drinks.
Have a great day, take some photos, breathe the fresh air and enjoy the area.