The Yanchep National Park is just north of Perth. Certainly, they must have had their own reasons to make this area a national park - for us though, it was not really clear why. But let me explain:
One of these reasons must be the shape of the terrain, which was formed over the years. But it isn't really spectactular. In the park, there are some caves, which you can enter only with Rangers during the guided tours. These caves are also rented as special venues for private events - so nothing for backpackers either.
If you decide to visit this park, then you should be aware that it will not be an adventure trip, but a relaxing day in the countryside by a lake. Residents of the nearby city of Perth use the park as a weekend destination and on Saturdays, you can also see a wedding party here to end the celebration.
In the park there are some barbeques. Kangaroos have already lost their fear of humans and are easier to find than to hide from them. The park also has a koala habitat. Koalas can be watched "wildly" here. They are not quite as wild as they are fed and they are only kept in a certain part of the park.
The park is perfect for a leisurely, hassle-free day, including some Australian animals and BBQ, before moving on to hectic Perth or saying goodbye to Perth. We went there, because the entrance was included with our Pinnacle Desert visit. Ask at the Pinnacle Entrance - if it's included it's definitely worth to spend a few chilling hours there, but I wouldn't pay just for the park.