Hit the low Isles with "Wavedancer" from Port Douglas. Technically I think it counts as the inner Barrier Reef. It doesn't have the 2 hours each way to get there and is very kid-friendly (and with amazing fish, turtles etc to see). One of the best, well organised excursions we have done.
https://wavedancerlowisles.com/
From Airlie Beach, we took a day trip to Daydream Island and that was a great day out. Snorkeling there too.
Singapore there is just so much to see and do. We had a week there and only scratched the surface. Are heading back there end of June for more 🙂