Confused....is your tour to the GBR departing from Sidney? Or are you spending the night before in Cairns? It has been 10 years since I did a snorkeling day on the reef, which was fabulous. About 20 of us on a large cat, took about 1.5 hours, leaving at 8.00 am to reach the atoll - only one other boat there, a private yacht. Be aware there are two sorts of GBR tours, small ones like mine or the large party boats, who all seem to congregate around one or two islands. About half an hour for a safety talk and instruction for those who had not snorkeled before, we swam for a couple of hours then took a lunch break, and swam some more until it was time to return. Did not put my damp T-shirt on after lunch, big mistake...got sunburnt, luckily not badly! So you get back to Cairns around 4.00, hopefully you can grab a shower somewhere at the docks/marina, then you have to get out to the airport which is outside of town, so say 3 hours with security clearance, plus 3 hour flight time to Sydney,, plus time to get back to where you are staying. So you are back in Sydney at around 11pm. I hardly think you will be ready to go out on the town after such a day! I’d much rather spend another night in Cairns. It’s a tourist oriented place so lots to do in the evening...good restaurants, pubs etc. Very early one morning I took a hot air balloon ride from Kuranda, stunning as the sun came up, and rode the tramway back down. Didn’t make it to Daintree, but wish I had. Visit the large mangrove swamp between the city and the airport, eerie, hot, intriguing, scary, home to various species of snakes and crocs.