It depends how much time you want to spend on the reef. You can do a day trip and see it from a pontoon or glass bottomed boat, or see it up closer by snorkeling or diving - and it's very rewarding if you do. You can also do a multiple-day trip with accommodation either onboard or at Reefworld/Reefsleep.
Another option is to take a helicopter flight that will give you some amazing panoramic views of the reef - and the Whitsundays - though of course you won't get the same up-close look that you will by boat.
This linked thread has more information on visiting the
Great Barrier Reef, hope this helps.
