just drink beer i live in the bay also,
you want to hit central station lake mckenzie, eurong, maheno ship wreck on the back beach, eli creek is really nice. you can do all of this in a day but to really enjoy your self id look to have 2 to 3 day's on the island, as there are plenty of places to go and see.
camping at orchid beach is really nice, no back packer's there not allowed up there.
up that far you can go to the champagne pools and indian head. if you decide to go futher up up around the sandy cape is really nice but watch your tide's and it's probably adviseable if you had another truck with you, cause in the inland track around nagarala rock's is very soft sand but the view is worth it.
if you get on ther ferry in hervey bay your looking $130 return and that's your vehicle permit as well, if you get on down at inskip point it's like 30 dollar's but you start at the bottom of the island and it is a sh!t of a drive to eurong.
if ya can afford the extra coin come to the bay it's really nice here. and im sure me or big col could show you some 4wd tracks around the bay.