I love the Sunsine Coast. You could consider Caloundra or Noosa or both if you’re doing a self drive. Also visit the hinterland around Montville and Maleny. You could continue further north to the Fraser Coast after the Sunshine Coast.
Snorkeling - some snorkeling to be had at Noosa Heads but it is not the focus there. Plenty of water activities though - surfing, kayaking, Everglades tours, SUP, hiking in the NP and beautiful beaches.
I Love Caloundra too.
We aren't too interested in the City, maybe spend a day or so there, but our departure flight is out of Brisbane, so depending on cost, because from what i've seen most car rentals have a one way fee. I'm open to flying into another airport. Would like to get a day or two in or snorkeling if we can manage finding decent spots. Otherwise we want to try surfing... (beginners:) and see some beautiful beaches and scenery.