There's not too much to do on Rottnest Island, just off the coast of Perth in Western Australia, unless you want there to be!
Since there aren't cars on the island, excepting the off maintenance vehicle and one bus, visitors use their two feet or a bike to get around. You can bring your own, or rent one. Since the island isn't large, you can't really get lost. Since there's very little infrastructure, there isn't much in the way of distraction from the pristine ocean water, white-sand beaches and incredible snorkeling that exists all over the islands surrounding reefs and limestone shelves.
My recommendation: take the ferry from Perth or Fremantle and plan to stay one night at minimum. The first day, bike the entire circumference of the island just to get your bearings. Do so without stopping very much, as long as you gear up with extra water, then tucker yourself out and get a good night's sleep. (Campsites and small cottages are both available to rent.) The next day, walk or ride the bus to the spots which you note to have the best snorkeling or the least-crowded stretches of sand. Do nothing at those places except sunbathe, bathe in the water or snorkel. Simply, relax.
It's incredible how so close to a major city it can feel like you are truly in the middle of nowhere and about as far from civilization as one can get without it being too far away to be useful in an emergency...
One last tip: don't miss out on enjoying a scoop or two at Simmo's Ice Cream.