Well I do a lot of hiking and notice that in Winter everyone starts to head off to the mountains for hiking, while summer is time to go to the beach.
Sometimes you can combine both, such as hiking at Noosa Heads http://www.weekendnotes.com/noosa-heads-hiking/ where you can hike through the bushland to many great beaches.
Of course there is also great walks and hiking on North Stradbroke Island and Bribie Island.