Rottnest island is only 25 minutes by ferry from Fremantle and is one of the most beautiful islands in the world with its white sandy beaches, crystal clear blue waters and unique wildlife its a must-do while in Perth.
Rottnet island has bike tracks running pretty much the whole way around the island so taking a bike is a great way to explore Rottnest and its pristine beaches.
There are two sets of shops on the island; There's the main shops near Thompson's (the bay where the ferries land) and then shops between Geordie and Longreach bays (known as the Geordie bay shops). Both sets of shops on Rottnest have a fully stocked supermarket type shops complete with Bottle shop. The prices in the Rottnest shops are a little more expensive than the mainland but not extortionate like other resort islands around the world. It's possible to get yourself bread and lunch meats to make sandwiches on your adventures at a reasonable price.
Both these sets of shops are on the one side of the island though so if you're going to make an adventure out of your day at Rottnest and make it to see the seals than you should pack supplies as there is no fresh water or food available outside of these areas.
If you don't have your own snorkel gear be sure to hire snorkel gear for your day as Rottnest's numerous bays and coves have some great fish and wildlife to see around the rocks.
While on Rottnest island you'll see lots of "Quokka"s which are like a smaller cousin of a Kangaroo/Wallaby which are super cute and great for photos if you haven't seen them before.
There's a nice bar on the island at "Thomson's" (the area where the Rottnest Ferry's come in) so after a day's exploring you can grab a drink before getting the ferry back in to town.
Ferries to and from the island depart multiple times a day and tickets can generally be bought the day before departure so it can be great for a last minute Perth holiday activity.