Noosa Heads, National Park. Walk as far as Hells gate, and then turn back. The track length is 5.4km if you go all the way to the end (one-way). Swim after your walk. The main beach is family friendly, allow at least 3 to 4 hours at Noosa heads. It takes approximately 1 hour and 50 minutes from Brisbane.
Fraser Island, hire a 4WD or go on a tour. Fraser island is a great camping, fishing and 4WD destination (Duration 2 days).