Best to sort something before you go, I've been all over the world and Perth is the only place where I've arrived with nothing booked and really really struggled to find a bed for the night. As in every single backpackers was full. Every motel was full. every pub was full. We found somewhere really really dodgy to stay (was on honeymoon with the wife) and then Seabreezer Bingles offered to let us stay with him, saved the day completely.
not all beaches around perth are very easily accessable with public transport. I'd say stay in Freo or the city and rent a car so you can go to all of the beaches around the place.
However:
There is a caravan park near woodman point called 'woodman point holiday park' that would be in walking distance.
If you would want to stay near Melville which is more in between freo and the city (both very accessible with public transport from here) you can stay at the canning bridge Motel or the raffles (the raffles is fairly expensive though) However Melville is a river location and gets crazy busy over the weekends. Not too bad through the week though.
Don;t know much about accomodation near al the other beaches but surely there would be hotels near leighton and stuff
