If you are in Perth around the dates of 7th - 9th April, head to Fremantle for the Fremantle Street Arts Festival 2012. It is the biggest street arts (buskers) festival in Australia and a great opportuinity for photos. Was there for it 4 years ago and flying there again in 2 weeks time just for this!! Cheers!
Fremantle Street Arts Festival - City of Fremantle
Caversham Wildlife Park is definately worth a visit too.
Nothing to worry about finding food and water, except on public holidays like Good Friday & Christmas when almost EVERYTHING is closed.
I take it that you are driving from the lists of places you are going, always top up your tank whenever you get the chance to, petrol prices rise slightly the further out from the city you are. And if you plan to cover long distances, load up on lots of energy drinks/coffee in the car, driving in aussie can be pretty mundane sometimes. (my mates and i drove 1600km non-stop(except to top up the tank) from Margeret River to Exmouth some years back when a storm came by and we had to change plans). Lastly, try to avoid driving in the dark on rurals roads as there is a very high chance of hitting animals.
Accomodation - i usually stay in backpacker hostels when travelling in Aust as i feel that they are more value for money if you are not travelling with family or kids, considering that accomodation in Perth is more expensive than the other cities. But as a tip, the Parmelia Hilton Hotel Perth is having a special rate of AUD$111/night the last i saw, which is DIRT CHEAP by aussie standards.
Hope you have a great time in Perth, Fremantle is one of my favourite cities in the world. Looking forward to my trip too.