Been there last Nov and love it.
You may also want to consider Bartholomew's Meadery - best honey farm and they sell various kind of honey and also must try honey wine...
At Fremantle - you may also want to go visit Fremantle prison. Also dont forget Fremantle Market only open on Weekend so take note. Lastly dont miss out eating nice sea food and best fish and chip in Fremantle. The 2 famous one are Kailis Fish Market Cafe and Cicerellos.
Lastly at Fremnatle try out this famous breakfast cafe shop call Gino's Cafe.
My advise is during touring Perth City and Fremantle you can go by train and foot thus save some money on car rental and parking.
At Perth, should go try out the best pizza ever, Little Caesars Pizzeria and Ciao Italia - Italian Restaurant Perth
If you go to Albany, you could take a look at this motel call Emu Point Motel & Apartments, best Service and friendly staffs I have ever met.
Whale world is a must to visit in Albany and also wind farm.
If you stay at Emu Point Motel & Apartments, behind it got a nice quiet bleach which you can take photo and play there.
Caversham Wildlife Park is nice but there is another which may also consider cause the park allow people to freely touch their animal and animal foods are provided for you to feed them, it is call Pentland Alpaca Stud & Tourist Farm
When at Cervantes you wont have must choice of place to stay but I would recommend Cervantes Pinnacles Caravan Park for nice friendly and services staff and cheap price.
Touring Pinnacles will seem boring and meaningless unless you get a local guide whom could tell you the history and also hidden and best spot to take photo. You could look for Mike which is my local guide at
The Pinnacles Western Australia
Lastly my advise is look at the sun rise and sun set timing (
Perth's Sunrise and Sunset Times) as this will allow you to plan your driving distance and when to set off. As everyone advise pls do not drive during dusk or early morning as it is very dangerous.
As for petrol price, whenever you pass by petrol station there is a big sign board telling you the petrol price, look out for it and you will get to know the general price is and which one will be cheaper. General thumb of rule is petrol cost slightly cheaper in city/town but when at rural price will go up.
Last of all remember to bring UV cream..