Rottnest Island is an amazing place to explore for a day or weekend. I would recommend a day trip if you are on a budget as the accommodation can become costly during peak times. The daily ferries depart in the morning and return late afternoon. I would recommend booking a fast ferry and going on a sunny warm day. Once you arrive you are spoilt for choice when deciding where to snorkel there are so many lovely coves and beach options with them all being surrounded by sandy shores and crystal clear water. The island has no cars so you can cycle around on hired bikes or bring your own via the ferry. There is plenty of sea life to explore and with the warm temperatures in summer the water is refreshing after a leisurely bike around the island.